Frame formats for distributed mimo

ABSTRACT

Disclosed herein are related to systems and methods for a multiple-input multiple-output (MIMO) communication. In one aspect, during a first time period, a master access point transmits, to a slave access point, information for a joint transmission by the master access point and the slave access point. In one aspect, the slave access point estimates synchronization information for the joint transmission, according to the information for the joint transmission. In one aspect, during a second time period after the first time period, the master access point transmits a portion of a null data packet to a station device. In one aspect, during the second time period, the slave access point transmits the portion of the null data packet to the station device, based on the synchronization information for the joint transmission. In one aspect, the station device determines steering information for the MIMO communication, according to the null data packet.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/724,294, filed Aug. 29, 2018, U.S. Provisional Patent ApplicationNo. 62/805,821, filed Feb. 14, 2019, and U.S. Provisional PatentApplication No. 62/828,874, filed Apr. 3, 2019, all of which areincorporated by reference in their entireties for all purposes.

FIELD OF THE DISCLOSURE

This disclosure generally relates to systems and methods for providing ajoint transmission by a group of distributed access points, includingbut not limited to frame formats of a joint transmission for soundingand steering.

BACKGROUND OF THE DISCLOSURE

In the last few decades, the market for wireless communications deviceshas grown by orders of magnitude, fueled by the use of portable devices,and increased connectivity and data transfer between all manners ofdevices. Digital switching techniques have facilitated the large scaledeployment of affordable, easy-to-use wireless communication networks.Furthermore, digital and radio frequency (RF) circuit fabricationimprovements, as well as advances in circuit integration and otheraspects have made wireless equipment smaller, cheaper, and morereliable.

Wireless communication can operate in accordance with various standardssuch as IEEE 802.11x, Bluetooth, global system for mobile communications(GSM), code division multiple access (CDMA). As higher data throughputand other changes develop, newer standards are constantly beingdeveloped for adoption, such as a progression from IEEE 802.11n to IEEE802.11ac.

SUMMARY

Various embodiments of a method for a multiple-input multiple-output(MIMO) communication are disclosed herein. In some embodiments, themethod includes transmitting, during a first time period, by a masteraccess point to a slave access point, information for a jointtransmission by the master access point and the slave access point. Insome embodiments, the method includes estimating, by the slave accesspoint, synchronization information for the joint transmission, accordingto the information for the joint transmission. In some embodiments, themethod includes transmitting, during a second time period after thefirst time period, by the master access point, a portion of a null datapacket to a station device. In some embodiments, the method includestransmitting, during the second time period, by the slave access point,the portion of the null data packet to the station device, based on thesynchronization information for the joint transmission. In someembodiments, the method includes determining, by the station device,steering information for the MIMO communication, according to the nulldata packet.

In some embodiments, estimating, by the slave access point, thesynchronization information includes estimating, by the slave accesspoint, a carrier frequency offset or a sampling frequency offset withrespect to the master access point. In some embodiments, the methodincludes transmitting, during a third time period before the first timeperiod, by the master access point, a slave trigger frame to the slaveaccess point. In some embodiments, the method includes estimating, bythe slave access point, additional synchronization information for anull data packet announcement, according to the slave trigger frame. Insome embodiments, the method includes transmitting, during a fourth timeperiod between the third time period and the first time period, by themaster access point, the null data packet announcement to the stationdevice. In some embodiments, the method includes transmitting, duringthe fourth time period, by the slave access point, the null data packetannouncement to the station device. In some embodiments, the methodincludes preparing, by the station device, for the null data packet, inresponse to receiving the null data packet announcement.

In some embodiments, transmitting, during the first time period, by themaster access point to the slave access point, the information for thejoint transmission, includes transmitting, by the master access point tothe slave access point, the information for the joint transmission in apreamble of the null data packet. In some embodiments, transmitting,during the first time period, by the master access point to the slaveaccess point, the information for the joint transmission, includestransmitting, by the master access point to the slave access point, theinformation for the joint transmission in a null data packetannouncement. In some embodiments, the method further includespreparing, by the station device, for the null data packet, in responseto the station device receiving the null data packet announcement. Insome embodiments, the method further includes transmitting, during athird time period between the first time period and the second timeperiod, by the master access point, a training field to the stationdevice. In some embodiments, the method further includes transmitting,during the third time period, by the slave access point, the trainingfield to the station device. In some embodiments, the method includesadjusting, by the station device, a gain setting to receive the portionof the null data packet, according to the training field.

Various embodiments of a first access point for a multiple-inputmultiple-output (MIMO) communication are disclosed herein. In someembodiments, the first access point includes a transceiver and asoundings controller. In some embodiments, the soundings controller isconfigured to cause, during a first time period, the transceiver totransmit, to a second access point, information for a joint transmissionby the first access point and the second access point. In someembodiments, the information for the joint transmission allows thesecond access point to estimate synchronization information for thejoint transmission. In some embodiments, the soundings controller isconfigured to cause, during a second time period after the first timeperiod, the transceiver to transmit a portion of a null data packet to astation device, based on the synchronization information for the jointtransmission. In some embodiments, the second access point transmits theportion of the null data packet to the station device. In someembodiments, the joint transmission of the portion of the null datapacket by the first access point and the second access point allows thestation device to determine steering information for the MIMOcommunication.

In some embodiments, the soundings controller is configured to cause thetransceiver to transmit the portion of the null data packet in atraining field. In some embodiments, the soundings controller is furtherconfigured to cause the transceiver to transmit, during a third timeperiod before the first time period, a slave trigger frame to the secondaccess point. In some embodiments, the slave trigger frame allows thesecond access point to estimate additional synchronization informationfor a null data packet announcement. In some embodiments, the soundingscontroller is further configured to cause the transceiver to transmit,during a fourth time period between the third time period and the firsttime period, the null data packet announcement to the station device,while the second access point transmits the null data packetannouncement to the station device during the fourth time periodaccording to the synchronization information. In some embodiments, thenull data packet announcement transmitted by the first access point andthe second access point allows the station device to prepare for thenull data packet. In some embodiments, the soundings controller isfurther configured to cause the transceiver to transmit the informationfor the joint transmission in a preamble of the null data packet.

In some embodiments, the soundings controller is further configured tocause the transceiver to transmit the information for the jointtransmission in a null data packet announcement. In some embodiments,the null data packet announcement allows the station device to preparefor the null data packet. In some embodiments, the soundings controlleris further configured to cause the transceiver to transmit, during athird time period between the first time period and the second timeperiod, a training field to the station device, while the second accesspoint transmits the training field to the station device during thethird time period. In some embodiments, the training field transmittedby the first access point and the second access point allows the stationdevice to adjust a gain setting to receive the portion of the null datapacket. In some embodiments, the soundings controller is furtherconfigured to cause the transceiver to transmit, during a third timeperiod between the first time period and the second time period, a nulldata packet announcement to the station device, while the second accesspoint transmits the null data packet announcement to the station deviceduring the third time period according to the synchronizationinformation. In some embodiments, the null data packet announcementtransmitted by the first access point and the second access point allowsthe station device to prepare for the null data packet. In someembodiments, the information for the joint transmission includes two ormore long training field (LTF) symbol groups that are separated by atleast a symbol. In some embodiments, the second time period isimmediately after the first time period.

Various embodiments disclosed herein are related to an access point fora multiple-input multiple-output (MIMO) communication. In someembodiments, the access point includes a transceiver and a soundingscontroller. In some embodiments, the soundings controller is configuredto cause the transceiver to receive, during a first time period, fromanother access point, information for a joint transmission by the accesspoint and the another access point. In some embodiments, the soundingscontroller is configured to estimate synchronization information for thejoint transmission, according to the information for the jointtransmission. In some embodiments, the soundings controller isconfigured to cause the transceiver to transmit, during a second timeperiod after the first time period, a portion of a null data packet to astation device according to the synchronization information, while theanother access point transmits the portion of the null data packet tothe station device during the second time period. In some embodiments,the portion of the null data packet transmitted by the access point andthe another access point allows the station device to determine steeringinformation for the MIMO communication.

In some embodiments, the soundings controller is configured to cause thetransceiver to transmit the portion of the null data packet in atraining field. In some embodiments, the soundings controller isconfigured to estimate the synchronization information for the jointtransmission by estimating a carrier frequency offset, a samplingfrequency offset, a first reference value for a common phase offset, ora second reference value for a timing offset with respect to the anotheraccess point.

In some embodiments, the soundings controller is further configured tocause the transceiver to receive, during a third time period before thefirst time period, from the another access point, a slave trigger frame.In some embodiments, the soundings controller is further configured toestimate additional synchronization information for a null data packetannouncement, according to the slave trigger frame. In some embodiments,the soundings controller is further configured to cause the transceiverto transmit, during a fourth time period between the third time periodand the first time period, the null data packet announcement to thestation device, while the another access point transmits the null datapacket announcement to the station device during the fourth time periodaccording to the synchronization information. In some embodiments, thenull data packet announcement transmitted by the access point and theanother access point allows the station device to prepare for the nulldata packet. In some embodiments, the soundings controller is configuredto cause the transceiver to receive the information for the jointtransmission in a preamble of the null data packet.

In some embodiments, the soundings controller is configured to cause thetransceiver to receive the information for the joint transmission in anull data packet announcement. In some embodiments, the null data packetannouncement allows the station device to prepare for the null datapacket. In some embodiments, the soundings controller is configured tocause the transceiver to transmit, during a third time period betweenthe first time period and the second time period, a training field tothe station device, while the another access point transmits thetraining field to the station device during the third time period. Insome embodiments, the training field transmitted by the access point andthe another access point allows the station device to adjust a gainsetting to receive the portion of the null data packet. In someembodiments, the soundings controller is configured to disable thetransceiver during a third time period between the first time period andthe second time period for a short interframe space. In someembodiments, the soundings controller is configured to cause thetransceiver to transmit, during a third time period between the firsttime period and the second time period, a null data packet announcementto the station device, while the another access point transmits the nulldata packet announcement to the station device during the third timeperiod according to the synchronization information. In someembodiments, the null data packet announcement transmitted by the accesspoint and the another access point allows the station device to preparefor the null data packet. In some embodiments, the second time period isimmediately after the first time period.

Various embodiments disclosed herein are related to a method for amultiple-input multiple-output (MIMO) communication. In someembodiments, the method includes transmitting, during a first timeperiod, by a master access point to a slave access point, informationfor a joint transmission by the master access point and the slave accesspoint. In some embodiments, the method includes causing, by the slaveaccess point, the slave access point to estimate synchronizationinformation for the joint transmission, according to the information forthe joint transmission. In some embodiments, the method includestransmitting, during a second time period after the first time period,by the master access point, a portion of a steered frame to a stationdevice. In some embodiments, the method includes transmitting, duringthe second time period, by the slave access point, the portion of thesteered frame to the station device, based on the synchronizationinformation for the joint transmission. In some embodiments, the methodincludes decoding, by the station device, the portion of the steeredframe transmitted by the master access point and the slave access pointto obtain content data in the portion of the steered frame.

In some embodiments, estimating, by the slave access point, thesynchronization information includes estimating, by the slave accesspoint, a carrier frequency offset or a sampling frequency offset withrespect to the master access point. In some embodiments, the informationfor the joint transmission is transmitted in a slave trigger frameduring the first time period.

In some embodiments, the steered frame transmitted by the master accesspoint includes a mid-amble. In some embodiments, the method includesbypassing, by the slave access point, a transmission, while the masteraccess point transmits the mid-amble of the steered frame. In someembodiments, the steered frame transmitted by the master access pointincludes a mid-amble. In some embodiments, the method further includesresynchronizing, by the slave access point, for another jointtransmission of another portion of the steered frame by the masteraccess point and the slave access point, according to the mid-amble.

In some embodiments, the method includes bypassing, during a third timeperiod between the first time period and the second time period, by themaster access point, a transmission for a short interframe space. Insome embodiments, the method includes transmitting, during a third timeperiod between the first time period and the second time period, by themaster access point, a training field to the station device. In someembodiments, the method includes transmitting, during the third timeperiod, by the slave access point, the training field to the stationdevice. In some embodiments, the method includes adjusting, by thestation device, a gain setting to receive the portion of the steeredframe, according to the training field.

In some embodiments, the method includes transmitting, during a thirdtime period after the first time period and the second time period, bythe master access point to the slave access point, a null data packet.In some embodiments, the method includes resynchronizing, during thethird time period, by the slave access point, for another jointtransmission of another steered frame by the master access point and theslave access point. In some embodiments, the method includestransmitting, during a fourth time period after the third time period,by the master access point, the another steered frame to the stationdevice. In some embodiments, the method includes transmitting, duringthe fourth time period, by the slave access point, the another steeredframe to the station device. In some embodiments, the method includesdecoding, by the station device, the another steered frame transmittedby the master access point and the slave access point to obtainadditional content data in the another steered frame. In someembodiments, the method includes transmitting, during a fifth timeperiod between the second time period and the third time period, by thestation device to the master access point and the slave access point, anacknowledgement frame. In some embodiments, the method includesscheduling, by the master access point and the slave access point, theanother joint transmission of the another steered frame, in response tothe acknowledgement frame.

Various embodiments disclosed herein are related to a first access pointfor a multiple-input multiple-output (MIMO) communication. In someembodiments, the first access point includes a transceiver and asteering controller. In some embodiments, the steering controller isconfigured to cause the transceiver to transmit, during a first timeperiod, to a second access point, information for a joint transmissionby the first access point and the second access point. In someembodiments, the information for the joint transmission allows thesecond access point to estimate synchronization information for thejoint transmission. In some embodiments, the steering controller isconfigured to cause the transceiver to transmit, during a second timeperiod after the first time period, a portion of a steered frame to astation device, while the second access point transmits the portion ofthe steered frame to the station device according to the synchronizationinformation. In some embodiments, the joint transmission of the portionof the steered frame by the first access point and the second accesspoint allows the station device to receive the portion of the steeredframe and decode the portion of the steered frame to obtain content datain the portion of the steered frame. In some embodiments, the secondtime period is immediately after the first time period.

In some embodiments, the information for the joint transmission istransmitted in a slave trigger frame during the first time period. Insome embodiments, the steered frame transmitted by the first accesspoint includes a mid-amble. In some embodiments, the second access pointis configured to bypass a transmission, while the first access pointtransmits the mid-amble of the steered frame. In some embodiments, thesteered frame transmitted by the first access point includes amid-amble. In some embodiments, the mid-amble allows the second accesspoint to resynchronize for another joint transmission of another portionof the steered frame by the first access point and the second accesspoint. In some embodiments, the mid-amble allows the second access pointto transition, during the transmission of the mid-amble by the firstaccess point, from a transmit mode to a receive mode and transition backto the transmit mode for the another joint transmission of the anotherportion of the steered frame by the first access point and the secondaccess point.

In some embodiments, the steering controller is configured to cause thetransceiver to bypass, during a third time period between the first timeperiod and the second time period, a transmission for a short interframespace. In some embodiments, the steering controller is configured tocause the transceiver to transmit, during the second time period, atraining field to the station device, while the second access pointtransmits the training field to the station device during the secondtime period. In some embodiments, the training field transmitted by thefirst access point and the second access point allows the station deviceto adjust a gain setting to receive the portion of the steered frame.

In some embodiments, the steering controller is configured to cause thetransceiver to transmit, during a third time period after the first timeperiod and the second time period, a null data packet to the secondaccess point. In some embodiments, the null data packet allows thesecond access point to resynchronize for another joint transmission ofanother steered frame by the first access point and the second accesspoint. In some embodiments, the steering controller is configured tocause the transceiver to transmit, during a fourth time period after thethird time period, the another steered frame to the station device,while the second access point transmits the another steered frame to thestation device during the fourth time period after the resynchronizing.In some embodiments, the another steered frame transmitted by the firstaccess point and the second access point allows the station device toreceive the another steered frame and decode the another steered frameto obtain additional content data in the another steered frame. In someembodiments, the steering controller is configured to cause thetransceiver to receive, during a fifth time period between the secondtime period and the third time period, from the station device anacknowledgement frame. In some embodiments, the steering controller isconfigured to schedule the another joint transmission of the anothersteered frame, in response to the acknowledgement frame. In someembodiments, the steering controller is configured to cause thetransceiver to transmit a unique pilot sequence associated with thefirst access point to enable the station device to estimate additionalsynchronization information, and receive the additional synchronizationinformation as a feedback.

Various embodiments disclosed herein are related to an access point fora multiple-input multiple-output (MIMO) communication. In someembodiments, the access point includes a transceiver and a steeringcontroller. In some embodiments, the steering controller is configuredto cause the transceiver to receive, during a first time period, fromanother access point, information for a joint transmission by the accesspoint and the another access point. In some embodiments, the steeringcontroller is configured to estimate synchronization information for thejoint transmission, according to the information for the jointtransmission. In some embodiments, the steering controller is configuredto cause the transceiver to transmit, during a second time period afterthe first time period, a portion of a steered frame to a station deviceaccording to the synchronization information, while the another accesspoint transmits the portion of the steered frame to the station deviceduring the second time period. In some embodiments, the portion of thesteered frame transmitted by the access point and the another accesspoint allows the station device to receive the portion of the steeredframe and decode the portion of the steered frame to obtain content datain the portion of the steered frame.

In some embodiments, the steering controller is configured to estimatethe synchronization information for the joint transmission by estimatinga carrier frequency offset, a sampling frequency offset, a firstreference value for a common phase offset, or a second reference valuefor a timing offset with respect to the another access point. In someembodiments, the steering controller is configured to cause thetransceiver to receive, during the first time period, from the anotheraccess point, the information for the joint transmission in a slavetrigger frame. In some embodiments, the second time period isimmediately after the first time period. In some embodiments, thesteering controller is configured to determine the synchronizationinformation according to a change in a phase offset or a timing offsetbetween a sounding sequence and a steering sequence. In someembodiments, the steered frame transmitted by the another access pointincludes a mid-amble. In some embodiments, the steering controller isconfigured to cause the transceiver to bypass a transmission, while theanother access point transmits the mid-amble of the steered frame. Insome embodiments, the steering controller is configured to, during thetransmission of the mid-amble by the another access point, resynchronizefor another joint transmission of another portion of the steered frameby the access point and the another access point and transition, from atransmit mode to a receive mode and transition back to the transmit modefor the another joint transmission of the another portion of the steeredframe by the access point and the another access point. In someembodiments, the steering controller is configured to cause thetransceiver to transmit, during the second time period, a training fieldto the station device, while the another access point transmits thetraining field to the station device during the second time period. Insome embodiments, the training field transmitted by the access point andthe another access point allows the station device to adjust a gainsetting to receive the portion of the steered frame. In someembodiments, the steering controller is configured to cause thetransceiver to receive, during a third time period after the first timeperiod and the second time period, a null data packet from the anotheraccess point. In some embodiments, the null data packet allows theaccess point to resynchronize for another joint transmission of anothersteered frame by the access point and the another access point. In someembodiments, the steering controller is configured to cause thetransceiver to transmit, during a fourth time period after the thirdtime period, the another steered frame to the station device after theresynchronizing, while the another access point transmits the anothersteered frame to the station device during the fourth time period. Insome embodiments, the another steered frame transmitted by the accesspoint and the another access point allows the station device to receivethe another steered frame and decode the another steered frame to obtainadditional content data in the another steered frame. In someembodiments, the steering controller is configured to cause thetransceiver to receive, during a fifth time period between the secondtime period and the third time period, from the station device anacknowledgement frame. In some embodiments, the steering controller isconfigured to cause the transceiver to transmit a unique pilot sequenceassociated with the access point to enable the station device toestimate additional synchronization information, and receive theadditional synchronization information as a feedback.

In some embodiments, the steered frame transmitted by the another accesspoint includes a mid-amble. In some embodiments, the steering controlleris configured to cause the transceiver to bypass a transmission, whilethe another access point transmits the mid-amble of the steered frame.In some embodiments, the steering controller is configured to cause thetransceiver to resynchronize for another joint transmission of anotherportion of the steered frame by the access point and the another accesspoint according to the mid-amble.

In some embodiments, the steering controller is configured to cause thetransceiver to bypass, during a third time period between the first timeperiod and the second time period, a transmission for a short interframespace. In some embodiments, the steering controller is configured tocause the transceiver to transmit, during a third time period betweenthe first time period and the second time period, a training field tothe station device, while the another access point transmits thetraining field to the station device during the third time period. Insome embodiments, the training field transmitted by the access point andthe another access point allows the station device to adjust a gainsetting to receive the portion of the steered frame.

In some embodiments, the steering controller is configured to schedulethe another joint transmission of the another steered frame, in responseto the acknowledgement frame.

BRIEF DESCRIPTION OF THE DRAWINGS

Various objects, aspects, features, and advantages of the disclosurewill become more apparent and better understood by referring to thedetailed description taken in conjunction with the accompanyingdrawings, in which like reference characters identify correspondingelements throughout. In the drawings, like reference numbers generallyindicate identical, functionally similar, and/or structurally similarelements.

FIG. 1A is a block diagram depicting a network environment including oneor more access points in communication with one or more devices orstations, according to some embodiments.

FIGS. 1B and 1C are block diagrams depicting computing devices useful inconnection with the methods and systems described herein, according tosome embodiments.

FIG. 2A is a block diagram depicting a system for performing channelestimation between a beamformer and a beamformee, according to someembodiments.

FIG. 2B is a block diagram depicting a system for performing channelestimation between at least one beamformer and at least one beamformee,according to some embodiments.

FIG. 3A is a block diagram illustrating an example network environmentincluding a master access point and multiple station devices for a jointtransmission, according to some embodiments.

FIG. 3B is a flow chart illustrating an example process of establishingan example multiple-input multiple-output (MIMO) communication,according to some embodiments.

FIG. 4 illustrates an example timing diagram of a joint transmission bya master access point and a slave access point to a station device.

FIGS. 5-9 illustrate example timing diagrams of a sounding sequence,according to some embodiments.

FIGS. 10-13 illustrate example timing diagrams of a steering sequence,according to some embodiments.

The details of various embodiments of the methods and systems are setforth in the accompanying drawings and the description below.

DETAILED DESCRIPTION

The following IEEE standard(s), including any draft versions of suchstandard(s), are hereby incorporated herein by reference in theirentirety and are made part of the present disclosure for all purposes:IEEE P802.11n™; and IEEE P802.11ac™. Although this disclosure canreference aspects of these standard(s), the disclosure is in no waylimited by these standard(s).

For purposes of reading the description of the various embodimentsbelow, the following descriptions of the sections of the specificationand their respective contents can be helpful:

-   -   Section A describes a network environment and computing        environment which can be useful for practicing embodiments        described herein; and    -   Section B describes embodiments of frame formats of a joint        transmission for sounding and steering in MIMO communication.

A. Computing and Network Environment

Prior to discussing specific embodiments of the present solution, it canbe helpful to describe aspects of the operating environment as well asassociated system components (e.g., hardware elements) in connectionwith the methods and systems described herein. Referring to FIG. 1A, anembodiment of a network environment is depicted. In brief overview, thenetwork environment includes a wireless communication system thatincludes one or more access points (APs) 106, one or more wirelesscommunication devices 102 and a network hardware component 192. Thewireless communication devices 102 can for example include laptopcomputers 102, tablets 102, personal computers 102, and/or cellulartelephone devices 102. The details of an embodiment of each wirelesscommunication device 102 and/or AP 106 are described in greater detailwith reference to FIGS. 1B and 1C. The network environment can be an adhoc network environment, an infrastructure wireless network environment,a subnet environment, etc., in one embodiment. The APs 106 can beoperably coupled to the network hardware 192 via local area networkconnections. The network hardware 192, which can include a router,gateway, switch, bridge, modem, system controller, appliance, etc., canprovide a local area network connection for the communication system.Each of the APs 106 can have an associated antenna or an antenna arrayto communicate with the wireless communication devices in its area. Thewireless communication devices 102 can register with a particular AP 106to receive services from the communication system (e.g., via a SU-MIMOor MU-MIMO configuration). For direct connections (e.g., point-to-pointcommunications), some wireless communication devices can communicatedirectly via an allocated channel and communications protocol. Some ofthe wireless communication devices 102 can be mobile or relativelystatic with respect to AP 106.

In some embodiments an AP 106 includes a device or module (including acombination of hardware and software) that allows wireless communicationdevices 102 to connect to a wired network using wireless-fidelity(WiFi), or other standards. An AP 106 can sometimes be referred to as awireless access point (WAP). An AP 106 can be implemented (e.g.,configured, designed and/or built) for operating in a wireless localarea network (WLAN). An AP 106 can connect to a router (e.g., via awired network) as a standalone device in some embodiments. In otherembodiments, an AP 106 can be a component of a router. An AP 106 canprovide multiple devices access to a network. An AP 106 can, forexample, connect to a wired Ethernet connection and provide wirelessconnections using radio frequency links for other devices 102 to utilizethat wired connection. An AP 106 can be implemented to support astandard for sending and receiving data using one or more radiofrequencies. Those standards, and the frequencies they use can bedefined by the IEEE (e.g., IEEE 802.11 standards). An AP 106 can beconfigured and/or used to support public Internet hotspots, and/or on anetwork to extend the network's Wi-Fi signal range.

In some embodiments, the access points 106 can be used for (e.g.,in-home or in-building) wireless networks (e.g., IEEE 802.11, Bluetooth,ZigBee, any other type of radio frequency based network protocol and/orvariations thereof). Each of the wireless communication devices 102 caninclude a built-in radio and/or is coupled to a radio. Such wirelesscommunication devices 102 and/or access points 106 can operate inaccordance with the various aspects of the disclosure as presentedherein to enhance performance, reduce costs and/or size, and/or enhancebroadband applications. Each wireless communication device 102 can havethe capacity to function as a client node seeking access to resources(e.g., data, and connection to networked nodes such as servers) via oneor more access points 106.

The network connections can include any type and/or form of network andcan include any of the following: a point-to-point network, a broadcastnetwork, a telecommunications network, a data communication network, anda computer network. The topology of the network can be a bus, star, orring network topology. The network can be of any such network topologyas known to those ordinarily skilled in the art capable of supportingthe operations described herein. In some embodiments, different types ofdata can be transmitted via different protocols. In other embodiments,the same types of data can be transmitted via different protocols.

The communications device(s) 102 and access point(s) 106 can be deployedas and/or executed on any type and form of computing device, such as acomputer, network device or appliance capable of communicating on anytype and form of network and performing the operations described herein.FIGS. 1B and 1C depict block diagrams of a computing device 100 usefulfor practicing an embodiment of the wireless communication devices 102or AP 106. As shown in FIGS. 1B and 1C, each computing device 100includes a central processing unit 121, and a main memory unit 122. Asshown in FIG. 1B, a computing device 100 can include a storage device128, an installation device 116, a network interface 118, an I/Ocontroller 123, display devices 124 a-124 n, a keyboard 126, and apointing device 127 such as a mouse. The storage device 128 can includean operating system and/or software. As shown in FIG. 1C, each computingdevice 100 can also include additional optional elements, such as amemory port 103, a bridge 170, one or more input/output devices 130a-130 n, and a cache memory 140 in communication with the centralprocessing unit 121.

The central processing unit 121 is any logic circuitry that responds toand processes instructions fetched from the main memory unit 122. Inmany embodiments, the central processing unit 121 is provided by amicroprocessor unit, such as: those manufactured by Intel Corporation ofSanta Clara, Calif., those manufactured by International BusinessMachines of White Plains, N.Y.; or those manufactured by Advanced MicroDevices of Sunnyvale, Calif. The computing device 100 can be based onany of these processors, or any other processor capable of operating asdescribed herein.

Main memory unit 122 can be one or more memory chips capable of storingdata and allowing any storage location to be directly accessed by themicroprocessor 121, such as any type or variant of Static random accessmemory (SRAM), Dynamic random access memory (DRAM), Ferroelectric RAM(FRAM), NAND Flash, NOR Flash and Solid State Drives (SSD). The mainmemory 122 can be based on any of the above described memory chips, orany other available memory chips capable of operating as describedherein. In the embodiment shown in FIG. 1B, the processor 121communicates with main memory 122 via a system bus 150 (described inmore detail below). FIG. 1C depicts an embodiment of a computing device100 in which the processor communicates directly with main memory 122via a memory port 103. For example, in FIG. 1C the main memory 122 canbe DRDRAM.

FIG. 1C depicts an embodiment in which the main processor 121communicates directly with cache memory 140 via a secondary bus,sometimes referred to as a backside bus. In other embodiments, the mainprocessor 121 communicates with cache memory 140 using the system bus150. Cache memory 140 typically has a faster response time than mainmemory 122 and is provided by, for example, SRAM, BSRAM, or EDRAM. Inthe embodiment shown in FIG. 1C, the processor 121 communicates withvarious I/O devices 130 via a local system bus 150. Various buses can beused to connect the central processing unit 121 to any of the I/Odevices 130, for example, a VESA VL bus, an ISA bus, an EISA bus, aMicroChannel Architecture (MCA) bus, a PCI bus, a PCI-X bus, aPCI-Express bus, or a NuBus. For embodiments in which the I/O device isa video display 124, the processor 121 can use an Advanced Graphics Port(AGP) to communicate with the display 124. FIG. 1C depicts an embodimentof a computer 100 in which the main processor 121 can communicatedirectly with I/O device 130 b, for example via HYPERTRANSPORT, RAPIDIO,or INFINIBAND communications technology. FIG. 1C also depicts anembodiment in which local busses and direct communication are mixed: theprocessor 121 communicates with I/O device 130 a using a localinterconnect bus while communicating with I/O device 130 b directly.

A wide variety of I/O devices 130 a-130 n can be present in thecomputing device 100. Input devices include keyboards, mice, trackpads,trackballs, microphones, dials, touch pads, touch screen, and drawingtablets. Output devices include video displays, speakers, inkjetprinters, laser printers, projectors and dye-sublimation printers. TheI/O devices can be controlled by an I/O controller 123 as shown in FIG.1B. The I/O controller can control one or more I/O devices such as akeyboard 126 and a pointing device 127, e.g., a mouse or optical pen.Furthermore, an I/O device can also provide storage and/or aninstallation medium 116 for the computing device 100. In still otherembodiments, the computing device 100 can provide USB connections (notshown) to receive handheld USB storage devices such as the USB FlashDrive line of devices manufactured by Twintech Industry, Inc. of LosAlamitos, Calif.

Referring again to FIG. 1B, the computing device 100 can support anysuitable installation device 116, such as a disk drive, a CD-ROM drive,a CD-R/RW drive, a DVD-ROM drive, a flash memory drive, tape drives ofvarious formats, USB device, hard-drive, a network interface, or anyother device suitable for installing software and programs. Thecomputing device 100 can further include a storage device, such as oneor more hard disk drives or redundant arrays of independent disks, forstoring an operating system and other related software, and for storingapplication software programs such as any program or software 120 forimplementing (e.g., configured and/or designed for) the systems andmethods described herein. Optionally, any of the installation devices116 could also be used as the storage device. Additionally, theoperating system and the software can be run from a bootable medium.

Furthermore, the computing device 100 can include a network interface118 to interface to the network 104 through a variety of connectionsincluding, but not limited to, standard telephone lines, LAN or WANlinks (e.g., 802.11, T1, T3, 56 kb, X.25, SNA, DECNET), broadbandconnections (e.g., ISDN, Frame Relay, ATM, Gigabit Ethernet,Ethernet-over-SONET), wireless connections, or some combination of anyor all of the above. Connections can be established using a variety ofcommunication protocols (e.g., TCP/IP, IPX, SPX, NetBIOS, Ethernet,ARCNET, SONET, SDH, Fiber Distributed Data Interface (FDDI), RS232, IEEE802.11, IEEE 802.11a, IEEE 802.11b, IEEE 802.11g, IEEE 802.11 n, IEEE802.1 lac, IEEE 802.11 ad, CDMA, GSM, WiMax and direct asynchronousconnections). In one embodiment, the computing device 100 communicateswith other computing devices 100′ via any type and/or form of gateway ortunneling protocol such as Secure Socket Layer (SSL) or Transport LayerSecurity (TLS). The network interface 118 can include a built-in networkadapter, network interface card, PCMCIA network card, card bus networkadapter, wireless network adapter, USB network adapter, modem or anyother device suitable for interfacing the computing device 100 to anytype of network capable of communication and performing the operationsdescribed herein.

In some embodiments, the computing device 100 can include or beconnected to one or more display devices 124 a-124 n. As such, any ofthe I/O devices 130 a-130 n and/or the I/O controller 123 can includeany type and/or form of suitable hardware, software, or combination ofhardware and software to support, enable or provide for the connectionand use of the display device(s) 124 a-124 n by the computing device100. For example, the computing device 100 can include any type and/orform of video adapter, video card, driver, and/or library to interface,communicate, connect or otherwise use the display device(s) 124 a-124 n.In one embodiment, a video adapter can include multiple connectors tointerface to the display device(s) 124 a-124 n. In other embodiments,the computing device 100 can include multiple video adapters, with eachvideo adapter connected to the display device(s) 124 a-124 n. In someembodiments, any portion of the operating system of the computing device100 can be configured for using multiple displays 124 a-124 n. Infurther embodiments, an I/O device 130 can be a bridge between thesystem bus 150 and an external communication bus, such as a USB bus, anApple Desktop Bus, an RS-232 serial connection, a SCSI bus, a FireWirebus, a FireWire 800 bus, an Ethernet bus, an AppleTalk bus, a GigabitEthernet bus, an Asynchronous Transfer Mode bus, a FibreChannel bus, aSerial Attached small computer system interface bus, a USB connection,or a HDMI bus.

A computing device 100 of the sort depicted in FIGS. 1B and 1C canoperate under the control of an operating system, which controlscheduling of tasks and access to system resources. The computing device100 can be running any operating system such as any of the versions ofthe MICROSOFT WINDOWS operating systems, the different releases of theUnix and Linux operating systems, any version of the MAC OS forMacintosh computers, any embedded operating system, any real-timeoperating system, any open source operating system, any proprietaryoperating system, any operating systems for mobile computing devices, orany other operating system capable of running on the computing deviceand performing the operations described herein. Typical operatingsystems include, but are not limited to: Android, produced by GoogleInc.; WINDOWS 7 and 8, produced by Microsoft Corporation of Redmond,Wash.; MAC OS, produced by Apple Computer of Cupertino, Calif.; WebOS,produced by Research In Motion (RIM); OS/2, produced by InternationalBusiness Machines of Armonk, N.Y.; and Linux, a freely-availableoperating system distributed by Caldera Corp. of Salt Lake City, Utah,or any type and/or form of a Unix operating system, among others.

The computer system 100 can be any workstation, telephone, desktopcomputer, laptop or notebook computer, server, handheld computer, mobiletelephone or other portable telecommunications device, media playingdevice, a gaming system, mobile computing device, or any other typeand/or form of computing, telecommunications or media device that iscapable of communication. In some embodiments, the computing device 100can have different processors, operating systems, and input devicesconsistent with the device. For example, in one embodiment, thecomputing device 100 is a smart phone, mobile device, tablet or personaldigital assistant. Moreover, the computing device 100 can be anyworkstation, desktop computer, laptop or notebook computer, server,handheld computer, mobile telephone, any other computer, or other formof computing or telecommunications device that is capable ofcommunication and that has sufficient processor power and memorycapacity to perform the operations described herein.

Aspects of the operating environments and components described abovewill become apparent in the context of the systems and methods disclosedherein.

B. Frame Formats for Sounding and Steering in MIMO Communication

Disclosed herein are related to a system and a method for providing ajoint transmission by a group (or plurality) of distributed accesspoints. In one aspect, disclosed herein are related to frame formats forsounding and steering of the joint transmission.

Described herein are systems and methods for establishing a MIMOconnection among two or more access points and at least a station device(sometimes referred to as a station or STA). In some embodiments, theaccess points include a master access point and two or more slave accesspoints capable of transmitting data through a wireless medium. In someembodiments, the master access point configures or coordinates withdifferent slave access points for a joint transmission. In one aspect, astation device may not be able to successfully receive and decodetransmission by a single master access point or a single slave accesspoint. However, the station device may successfully receive and decode ajoint transmission by two or more access points.

In some embodiments, the master access point configures differentstation devices for a joint transmission in two phases: a sounding and asteering. During the sounding, channel estimation is performed todetermine signal strengths, relative signal phases, and/or qualities oftransmissions from different access points received by station devices,and/or to determine a configuration of a joint transmission according tothe determined signal strengths, relative signal phases, or qualities oftransmissions from different access points. Examples of theconfiguration of the joint transmission can include timing, power, gain,channel bandwidth, frequency for transmission through one or moreantennas, steering vector, or any information indicating how to radiateenergy in a particular direction, frequency-dependency within a channel,signal-to-noise ratio (SNR), and/or appropriate data rate adjustments.During the steering phase, the access points and the station devices cancommunicate through a beamforming according to the configurationdetermined during the sounding, such that the station devices canreceive and decode joint transmissions from the access points.

Disclosed herein are related to frame formats for sounding and/orsteering. In one aspect, the frame formats disclosed herein can enableslave access points to estimate or determine synchronization informationfor joint transmission with a master access point. Examples of thesynchronization information can include a carrier frequency offset, asampling frequency offset, a first reference value for a common phaseoffset, and/or a second reference value for a timing offset with respectto the master access point. For example, the master access pointtransmits a slave trigger frame or a preamble of a null data packet thatenables slave access points to estimate or determine synchronizationinformation. According to the synchronization information, the masteraccess point and the slave access points may transmit togethersimultaneously for sounding, steering or both.

Described herein are systems and methods for performing channelestimation between a beamformer and a beamformee (e.g., at least one AP106 and at least one device 102) for example in a multi-usermultiple-input and multiple-output (MU-MIMO) environment. The AP 106(hereinafter sometimes generally referred to as an “access point” or“AP”), for example in a MU-MIMO configuration, can include an AP 106that can communicate with each of a plurality of devices 102 (e.g.,beamformees). The AP 106 can include a number of antennas and cansupport a number of spatial streams for transmission to a device(sometimes referred to as a station (STA) or user) 102. An AP 106 canleverage and use one or more sounding frames, such as null data packet(NDP) or NDP announcement (NDPA) frames or other control frames, torequest the device 102 for channel estimation feedback.

The dimension of the AP 106 (embodied as a beamformer) sometimes refersto the number of spatial streams or the number of available transmitantennas the AP 106 supports. The dimension of the wirelesscommunication device 102 (embodied as a beamformee) sometimes refers tothe number of spatial streams the device 102 can support, e.g., forchannel estimation. In some aspects, the present methods and systems canallow a higher dimension AP 106 or a coordinated set of APs 106 (bothhereafter sometimes generally referred to as a single “beamformer”,wherein the coordinated set of APs 106 can be configured to operate insome ways like a single beamformer), to use the beamformer'stransmission antennas to steer and achieve full array gain even ifpaired against a device 102 of lower dimension. The device 102 can becapable of supporting less than the number of spatial streams that theAP 106 can include in a sounding, for performing multi-inputmulti-output (MIMO) channel estimation. The present solution can providefor multiple soundings using subsets of the beamformer's antennas, inplace of a single sounding using all of the beamformer's antennas (whichthe beamformee does not support). A coordinated sequence of soundingsfrom subsets of antennas can elicit different sets of MIMO channelestimation results from a low dimensional beamformee, which can becombined to produce full channel estimation information. For example, incertain embodiments, by defining the subsets to have overlappingantennas, the channel estimation feedback matrices for the differentsubsets/soundings can be merged or combined to yield channel estimationfor the full channel of the AP 106.

In some embodiments, multiple beamformers (e.g., APs 106) can operate ina coordinated fashion as if a higher dimensional beamformer iscommunicating with one or more beamformee(s). Multiple sounding reportsfrom the same device 102 to the different APs 106 can be combined and/orshared between the coordinated APs 106, to understand the MU-MIMOchannels between each beamformer-beamformee pair. In another embodiment,the coordinated APs 106 can be time and/or frequency synchronized (e.g.,using frequency diversity or offset) to provide a sounding sequence thatcan appear like a single sounding sequence from a higher dimensionbeamformer.

In some embodiments, the AP 106, e.g., embodied as a beamformer, canreceive responses from the device 102, e.g., embodied as a beamformee,to the plurality of sounding frames. The responses can include channelestimation for the plurality of subsets of transmit spatial streams. TheAP 106 can generate full channel estimation information based on thereceived responses. The full channel estimation information can includechannel estimation information corresponding to all the transmit spatialstreams that the AP 106 can provide. The AP 106 can steer the pluralityof transmit antennas of the AP 106 based on the full channel estimationinformation.

In yet another aspect, this disclosure is directed to a method forperforming channel estimation between a plurality of APs 106 embodied asbeamformers and at least one device 102 embodied as a beamformee. Themethod can include configuring a first AP 106 and a second AP 106 tocommunicate with a first device 102. The first AP 106 can support afirst plurality of transmit spatial streams. The second AP 106 cansupport a second plurality of transmit spatial streams. The first AP 106and the second AP 106 can determine, between the first AP 106 and thesecond AP 106, a plurality of subsets of transmit spatial streams fromthe first plurality of transmit spatial streams and the second pluralityof transmit spatial streams. The first AP 106 and the second AP 106 cancoordinate, between the first and the second beamformers 106,transmission of a plurality of sounding frames to the first device 102for channel estimation based at least on the determined plurality ofsubsets of transmit spatial streams.

In some embodiments, the first AP 106 and the second AP 106 areconfigured to communicate with the first device 102. The first AP 106and the second AP 106 can be operably coupled via a backbone connection.The first AP 106 and/or the second AP 106 can combine at least someresponses from the first device 102 to the plurality of sounding frames,the responses including channel estimation for the plurality of subsetsof transmit spatial streams. The first AP 106 and/or the second AP 106can generate full channel estimation information between the first AP106 and the second AP 106, based on the combination of the at least someresponses. The first AP 106 and the second AP 106 can be configured tocommunicate with a plurality of beamformees 102. The first AP 106 andthe second AP 106 can coordinate, between the first AP 106 and thesecond AP 106, transmission of a plurality of sounding frames to aplurality of the devices 102. The first AP 106 and/or a second AP 106can use frequency diversity to simultaneously send a sounding frame fromthe first AP 106 and a sounding frame from the second AP 106.

Referring to FIG. 2A, an embodiment of a system for establishing a MIMOcommunication between at least one AP 106 and at least one device 102 isdepicted. In brief overview, one embodiment of the system can includethe AP 106 in communication with the device 102. The AP 106 can includea wireless transceiver 228, a soundings controller module (SCM) 222(also referred to as “a soundings controller 222”), a steeringcontroller module 225 (also referred to as “a steering controller 225”),an integration module 221 (also referred to as “an integrationcontroller 221”), a storage module 223 and/or a configuration 224. Thedevice 102 can include a wireless transceiver and a channel estimationmodule (CEM) 212 (also referred to as “a channel estimation controller212”). The AP 106 can include a plurality of antennas (e.g., phase arrayantennas).

In some embodiments, the system can support at least some aspects of aMU-MIMO transmission configuration 224 between the AP 106 embodied as abeamformer and the device 102 embodied as a beamformee. For example, theAP 106 can be of higher dimensionality relative to the device 102. Thedevice 102 can perform channel estimation for a number of spatialcommunication streams up to the dimensionality of the device 102. Device102 can provide channel estimation for more dimensions than it hasantennas in one embodiment. For example, device 102 can have only oneantenna but can have the capability to estimate a three antenna channel.By way of illustration, the AP 106 can be of higher dimensionalityand/or capability than some of the devices 102 (e.g., older generationbeamformees) in deployment. In one embodiment, the device 102 can forexample be capable of performing channel estimation for up to apre-configured number of transmit spatial streams from the AP 106, thepre-configured number of transmit spatial streams being less than anumber of transmit spatial streams that the AP 106 can provide.

The AP 106 can be implemented (e.g., configured and/or built) tosupport, simultaneously or otherwise, a first number of transmitstreams, channels or connections, for instance within a transmission orsounding frame. The AP 106 can include a coordinated set of APs 106. Thesounding frame can include an MU-MIMO frame, SU-MIMO frame and/or an NDPframe. The AP 106 can be implemented to include a first number oftransmit chains and/or transmit antennas. The device 102 can beimplemented to support (simultaneously or otherwise) a second number ofspatial streams, channels or connections (hereafter sometimes generallyreferred to as “spatial streams” or “Nss”). The second number of spatialstreams can be lower than the first number of streams.

In some embodiments, a total or predetermined number of spatial streamsthat a device 102 can support can be stored or indicated in thebeamformee's capability field—“Compressed Steering Number of BeamformerAntennas Supported.” This field can be defined or included in amanagement frame. This field can include or indicate a Nss that the AP106 can handle or support in an NDP frame, for example. This field canset the range of transmit spatial dimensions for which the device 102can perform channel estimation, generate a proper beamforming report,and/or create a sounding feedback frame (sometimes generally referred toas a “response”) to the AP 106. In some embodiments, this capability isfixed once the device 102 or AP 106 is implemented and/or deployed inthe field, and in some embodiments not upgradable (e.g., through asoftware patch).

In some embodiments, the pre-configured or ceiling Nss of the device 102can limit transmit beamforming performance, e.g., of the AP 106 and/orthe device 102. For example, when the device's channel estimationcapability fails to support the full extent of the AP's transmitantennas, the AP 106 might not be able to use all of its transmitantennas for steering, and might not be able to achieve full array gain.In some embodiments, for multi-user transmit beamforming, the AP 106might have to use the minimal sounding capability among the devices 102with different sounding capabilities. The AP 106 might have to turn onfewer than the full set of antennas, corresponding to the minimalsounding capability. Existing transmit-beamforming (TXBF) capabledevices with limited sounding capabilities can remain available in themarket and/or remain deployable in the field for some time, before thesedevices are eventually replaced by higher capability devices. Thepresent methods and systems, in some aspects, provide support for suchdevices 102.

In some embodiments, the AP 106 includes a transceiver 228 allowing thebeamformer 106 to communicate with other devices (e.g., beamformee 102or another beamformer 106) through one or more antennas. For example,the transceiver 228 includes a transmitter that encodes data (e.g.,content data, control data, response data, announcement, etc.) at abaseband frequency, upconverts the encoded data to a radio frequency,and transmits the upconverted signal through one or more antennas. Foranother example, the transceiver 228 includes a receiver thatdownconverts a wireless signal at a radio frequency received through oneor more antennas to a baseband frequency, and decodes the convertedsignal to obtain data (e.g., content data, control data, response data,announcement, etc.). In some embodiments, the AP 106 transmits andreceives data at the same frequency, separate frequencies, or atoverlapping frequencies.

In some embodiments, the AP 106 includes a SCM 222. The SCM 222 canmanage and/or control transmission of a plurality of sounding frames(e.g., NDP sounding frames) to a low-capability device 102. The SCM 222can include hardware, or a combination of hardware and software. Forexample, the SCM 222 can include any application, program, library,script, task, service, process or any type and form of executableinstructions executing on hardware of the AP 106. In one embodiment, theSCM 222 includes a set of executable instructions executing on a core orprocessor of the AP 106. The SCM 222 can include circuitry designedand/or constructed to perform any of the operations and functionsdescribed herein. In some embodiments, the SCM 222 is configured tocontrol transmission of sounding frames to the device 102 via one ormore antennas of the AP 106. For example, the SCM 222 can be configuredto allocate, select or assign certain antennas to participate in one ormore soundings within a set of soundings. The SCM 222 can for exampletune a subset of the beamformer's antennas (e.g., phase array antennas)to participate in a particular sounding. In some embodiments, the SCM222 of a master AP determines, instructs, or schedules, a soundingsequence, as disclosed herein. In some embodiments, the SCMs 222 ofdifferent APs coordinate with each other to determine or schedule thesounding sequence disclosed herein.

In some embodiments, the SCM 222 includes firmware executing on the AP106 hardware. The firmware can operate in a layer of a protocol stack ofthe AP 106 (e.g., in an upper layer). In certain embodiments, the SCM222 operates in the MAC layer, e.g., residing between a lower layer ofMAC and a higher layer of MAC. The AP 106 or SCM 222 can partition,allocate or otherwise assign the plurality of antennas into a pluralityof subsets of antennas, e.g., based on a configuration 224. In someembodiments, an antenna can be assigned to more than one of the subsets.Each subset of antennas can be selected according to a configuration224, which can be consistent with one or more predefined rules,policies, requirements and/or guidelines. For example, each subset ofantennas can include antennas physically adjacent to each other on theAP 106, and/or selected according to a certain order or sequence. Insome embodiments, each subset of antennas can include antennas that areseparated from each other by at least another antenna not in the subset,e.g., to include some spatial degree of freedom or separation. In someembodiments, the SCM 222 configures each subset to include at least oneantenna common to at least one other subset.

The SCM 222 can manage or control the transmission of a plurality ofsounding frames corresponding to the subsets of antennas. Each soundingcan cover a subset of the AP's antennas. The SCM 222 or AP 106 caninstruct, cause, or allow the coordinated transmission in soundingframes through different antennas in a particular order. The SCM 222 orAP 106 can cause the transceiver 228 to transmit the soundings framesdistributed or staggered over a period of time, e.g., at predefinedintervals. The SCM 222 or AP 106 can provide sufficient time for the AP106 to process each sounding frame and/or provide a response to eachsounding frame, before sending a next sounding frame. The SCM 222 canstore information about the configured subsets, and/or information abouta sequence of corresponding soundings, to the configuration 224 and/orstorage module 223.

The configuration 224 can include, maintain or store information aboutassignment or allocation of antennas to subsets for sounding purposes,and/or information about a sequence of soundings corresponding to theconfigured subsets. The configuration 224 can include information aboutconnecting or coupling a specific antenna to a particular transmitchain. The configuration 224 can include a schedule or timinginformation for performing soundings. The configuration 224 can bestored in a storage module 223. The storage module 223 can include oneor more interconnected storage devices, such as any embodiment ofstorage devices 128, 140, 122, described above in connection with FIGS.1B and 1C. In some embodiments, the SCM 222 and/or the integrationmodule can generate, access and/or update the configuration.

The configuration 224 can include a list, table or other databasestructure, and can include at least one entry, record or specificationcorresponding to each device 102 for example. The configuration 224 canbe specified and/or defined in a file or a collection of records, storedor maintained in the storage module 223. A SCM 222 and/or integrationmodule 221 can access a portion of the configuration, to implementpartial channel estimation through multiple soundings. For example, anintegration module can use a hash to perform lookup in the configuration224 for an antenna or device, to process channel estimation feedback.

In some embodiments, the device 102 includes a CEM 212. The CEM 212 canprocess a sounding frame received by the device 102. For example, theCEM 212 can perform channel estimation and/or provide channel estimationfeedback. Channel estimation can include any form of channel measurementand/or calibration, or channel response determination. Channelestimation results can be used to provide feedback to the AP 106 on howto radiate energy in a particular direction, frequency-dependency withina channel, signal-to-noise ratio (SNR), and/or appropriate data rateadjustments, for example. The CEM 212 can include hardware, or acombination of hardware and software. For example, the CEM 212 caninclude any application, program, library, script, task, service,process or any type and form of executable instructions executing onhardware of the CEM 212. In one embodiment, the CEM 212 includes a setof executable instructions executing on a core or processor of the CEM212.

The CEM 212 can include circuitry designed and/or constructed to performany of the operations and functions described herein. In someembodiments, the CEM 212 is pre-configured to perform channel estimationin a particular environment, e.g., a SU-MIMO or MU-MIMO environment. Insome embodiments, the CEM 212 includes firmware executing on thehardware of the device 102. The firmware can operate in a layer of aprotocol stack of the device 102 (e.g., in an upper layer). The CEM 212can operate in the MAC layer, e.g., residing between a lower layer ofMAC and a higher layer of MAC. The CEM 212 can provide the channelestimation feedback for inclusion in a response to a sounding frame fromthe AP 106. The CEM 212 can manage or direct the transmission of one ormore responses to the AP 106.

In some embodiments, the AP 106 can include an integration module 221.The integration module 221 can manage, track and/or process theresponses received from the device 102. The integration module 221 canstore information (e.g., channel estimation information) from one ormore responses in a storage module 223. The integration module 221 canorder and/or combine channel estimation information from the response,for example, based at least in part on information stored in theconfiguration 224 (e.g., information about the configured subsets,and/or the sequence of soundings corresponding to the configuredsubsets). According to the channel estimation information, theintegration module 221 may determine steering information. In oneaspect, the steering information indicates configuration of thetransceiver 228 of the AP 106 and/or other APs 106 for steering. Forexample, the steering information indicates timing, power, gain, channelbandwidth, frequency for transmission through one or more antennas,steering vector, or indicates how to radiate energy in a particulardirection, frequency-dependency within a channel, signal-to-noise ratio(SNR), and/or appropriate data rate adjustments for joint transmissionwith other access points.

The integration module 221 can include hardware, or a combination ofhardware and software. For example, the integration module 221 caninclude any application, program, library, script, task, service,process or any type and form of executable instructions executing onhardware of the AP 106. In one embodiment, the integration module 221includes a set of executable instructions executing on a core orprocessor of the AP 106. The integration module 221 can includecircuitry designed and/or constructed to perform any of the operationsand functions described herein. In some embodiments, the integrationmodule 221 includes firmware executing on the hardware of the AP 106.The firmware can operate in a layer of a protocol stack of the AP 106(e.g., in an upper layer). In certain embodiments, the integrationmodule 221 operates in the MAC layer, e.g., residing between a lowerlayer of MAC and a higher layer of MAC. In some embodiments, theintegration module 221 is configured to combine, integrate, merge,normalize, adjust and/or aggregate information from the responses toobtain full channel information. In some embodiments, the integrationmodule 221 is integrated with the soundings controller 222 or isimplemented as part of the soundings controller 222.

In some embodiments, the AP 106 includes a steering module 225. Thesteering module 225 can manage and/or control transmission of steeredframes to a low-capability device 102. The steering module 225 caninclude hardware, or a combination of hardware and software. Forexample, the steering module 225 can include any application, program,library, script, task, service, process or any type and form ofexecutable instructions executing on hardware of the AP 106. In oneembodiment, the steering module 225 includes a set of executableinstructions executing on a core or processor of the AP 106. Thesteering module 225 can include circuitry designed and/or constructed toperform any of the operations and functions described herein. In someembodiments, the steering module 225 is configured to controltransmission of steered frames to the device 102 via one or moreantennas of the AP 106. For example, the steering module 225 can beconfigured to allocate, select or assign certain antennas to participatein one or more steerings. The steering module 225 can for example tune asubset of the beamformer's antennas (e.g., phase array antennas) toparticipate in a particular steering.

In some embodiments, the steering module 225 includes firmware executingon the AP 106 hardware. The firmware can operate in a layer of aprotocol stack of the AP 106 (e.g., in an upper layer). In certainembodiments, the steering module 225 operates in the MAC layer, e.g.,residing between a lower layer of MAC and a higher layer of MAC. The AP106 or steering module 225 can partition, allocate or otherwise assignthe plurality of antennas into a plurality of subsets of antennas, e.g.,based on a configuration 224 or the integration module 221. In someembodiments, an antenna can be assigned to more than one of the subsets.Each subset of antennas can be selected according to a configuration 224and/or by the integration module 221, which can be consistent with oneor more predefined rules, policies, requirements and/or guidelines. Forexample, each subset of antennas can include antennas physicallyadjacent to each other on the AP 106, and/or selected according to acertain order or sequence. In some embodiments, each subset of antennascan include antennas that are separated from each other by at leastanother antenna not in the subset, e.g., to include some spatial degreeof freedom or separation. In some embodiments, the steering module 225configures each subset to include at least one antenna common to atleast one other subset.

The steering module 225 can manage or control transmission of data(e.g., content data) corresponding to the subsets of antennas. Thesteering module 225 can cause the coordinated transmission or jointtransmission of content data in steered frames through differentantennas or with another AP 106 in a particular order. The steeringmodule 225 may transmit according to the steering information (e.g.,steering vector). The steering module 225 can cause the transceiver 228to transmit the steered frames distributed or staggered over a period oftime, e.g., at predefined intervals. The steering module 225 can providesufficient time for another AP 106 or a device 102 to process eachsteering and/or provide a response to each steering, before sending anext steered frame.

In some embodiments, the present methods can be implemented or appliedtransparent to a low capability device 102. For example, the device 102might not even realize or be aware that the multiple access points 106are coordinated and transmit sounding frames or steered framessimultaneously.

In some embodiments, a high dimensional transmit structure (e.g., asingle AP 106) can be formed by combining and/or coordinating two ormore lower dimensionality beamformers 106. Referring to FIG. 2B, anembodiment of a system for performing sounding and steering between atleast one AP 106 and at least one device 102 is depicted. In briefoverview, the system can include at least two APs operably coupledtogether to effectively operate as a single AP 106 (hereafter sometimesreferred to as “coordinated beamformers” or as a single “beamformer”).The coordinated beamformers can be in communication with one or moredevices 102 embodied as beamformees. The coordinated beamformers caninclude a transceiver 228, a SCM 222, an integration module 221, asteering module 225, a storage module 223 and/or a configuration 224,each of which can include one or more features described above inconnection with FIG. 2A. Each of these modules can reside on at leastone of the coordinated beamformers (APs 106), distributed amongdifferent APs 106, or can reside at one or more networked locations.

In some embodiments, increasing or combining the number of antennas atthe AP 106 can provide better or improved array gain and diversity. Byway of illustration, for single-user transmit beamforming, an array gaincan be represented by 10*log 10(NTX/Nss) dB, and a correspondingdiversity order can be represented as NTX*NRX, where NTX can representthe number of transmit antennas, and NRX can represent the number ofreceive antennas. A large number of transmit antennas at the beamformerside can provide more spatial degrees of freedom, e.g., allowing forimproved link quality. In some embodiments, total capacity of the systemcan increase linearly with respect to the number of beamformer antennas(e.g., in an asymptotic sense). A larger number of transmit antennas cansupport or host more beamformees (e.g., multi-user STAs), for instanceusing a multi-user steered frame to achieve better spectrum efficiency.In certain embodiments, a larger number of transmit antennas can allowfor better multi-user precoding matrices for a given set of beamformees(e.g., multi-user STAs). A significant portion of existing access points106 can be implemented with a limited number of transmit antennas, e.g.,up to three antennas. In accordance with certain aspects of thedisclosure, transmit antennas from a few low dimensional beamformers 106can be combined or aggregated to achieve better performance.

In some embodiments, multiple lower dimensional APs 106 are connected toeach other, for example through an Ethernet backbone or other connection(e.g., a wireless connection such as Wi-Fi, cellular, Bluetooth, etc.).These APs 106 can be coordinated to generate multiple sounding sequencesbetween the APs 106 and the devices 102, to determine the channels(e.g., MU-MIMO channels) between each beamformer-beamformee pair.Sounding feedback reports from different devices 102 can be sharedand/or processed between the coordinated beamformers (e.g., APs 106),for example, using the techniques described in connection with at leastFIGS. 2A and 2B. For example, multiple sounding feedback reports fromthe same device 102 to multiple APs 106 can be combined in such a way asif the APs 106 include a single higher dimensional AP 106 communicatingto the device 102. Multi-user precoding can be performed, for example,using post-combined MU-MIMO channels. When performing multi-usersteering, the coordinated beamformers 106 might have to coordinate thesteered frame transmission so that the steered frame as formed appearsto come from a larger dimensional AP 106 rather than several differentlower dimensional APs 106. The coordinated APs 106 can be timesynchronized. For example, the APs 106 can coordinate steered frametransmission through a backbone Ethernet or other connection (e.g., awireless connection such as Wi-Fi, cellular, Bluetooth, etc.).

In certain embodiments, the system can time and/or frequency synchronizethe coordinated APs 106, for example by adjusting frame timing and/orcarrier frequency offset (CFO). The coordinated APs 106 can transmitsounding frames, steered frames, or a combination of them in acoordinated manner as if the frames are sent from a single largerdimensional AP 106. The precoding matrix and/or steered frame can beformed in a coordinated manner. When performing multi-user steering, thecoordinated APs 106 might have to coordinate the steered frametransmission so that the steered frame as formed appears to come from alarger dimensional AP 106 rather than several different lowerdimensional APs 106. In performing frequency synchronization, thecoordinated APs 106 can synchronize their carrier frequency offset orCFO. This can be performed by observing uplink transmission fromdifferent devices 102, sharing the CFO among the APs 106. In someembodiments, the coordinated APs 106 can be time synchronized. Forexample, one of the APs 106 can begin frame transmission first (e.g.,L-STF and L-LTF signals transmitted first). The rest of the coordinatedAPs 106 can join in the transmission starting from the VHT-STF signal,for example by monitoring for presence and/or timing of the L-STF andL-LTF signal on the air.

FIG. 3A is a block diagram illustrating an example network environment300 including a master AP 310 and multiple slave APs 320A, 320B for ajoint transmission to stations devices 330A-330E, according to someembodiments. In some embodiments, each of the master AP 310 and theslave APs 320A, 320B, or the combination of the master AP 310 and theslave APs 320A, 320B is implemented or operate as the beamformer 106 ofFIGS. 2A and 2B. In some embodiments, the network environment 300includes more, fewer or different components than shown in FIG. 3A.

In some embodiments, the master AP 310 is capable of directlycommunicating with the slave APs 320A, 320B, and the station devices330A, 330C through a wireless medium, but may be incapable of directlycommunicating with the station devices 330B, 330D, 330E alone. Forexample, the station devices 330A, 330C can successfully receive anddecode a wireless transmission by the master AP 310 alone, but thestation devices 330B, 330D, 330E may not successfully receive and decodethe wireless transmission by the master AP 310 alone. In someembodiments, the slave AP 320A is capable of directly communicating withthe master AP 310 and the station devices 330B, 330C, 330D. In someembodiments, the slave AP 320B is capable of directly communicating withthe master AP 310 and the station devices 330C, 330D, 330E.

In some embodiments, the master AP 310 configures, causes, or instructsthe slave APs 320A, 320B for a joint transmission. In one aspect, thestation devices 330A-330E can successfully receive and decode the jointtransmission by the master AP 310 and the slave APs 320A, 320B. In someembodiments, the master AP 310 and the slave APs 320A, 320B communicatewith each other through a wireless medium in a particular frame formatto enable a joint transmission. In one aspect, a frame format disclosedherein enables the slave APs 320A, 320B to estimate or determinesynchronization information for a joint transmission with the master AP310. Examples of the synchronization information include a carrierfrequency offset (CFO), a sampling frequency offset (SFO), a phaseoffset (PO), a timing offset (TO), a first reference value for a commonphase offset, and/or a second reference value for a timing offset withrespect to the master AP 310. For example, the master AP 310 transmits,to the slave APs 320A, 320B, a slave trigger frame or a preamble of anull data packet including information that enables the slave APs 320A,320B to estimate or determine synchronization information. In someembodiments, the synchronization information is determined according toa difference of measurements obtained in the sounding sequence and thesteering sequence. In one approach, the slave AP uses the channelestimations, denoted here by the vectors h_ndp (for the channel beforethe NDP) and h_data (for the channel before the joint data transmission)to calculate two parameters related to the phase drift: timing errors(or acquisition error) and a common phase drift. In some embodiments,the timing errors are manifested as a linear phase drift acrossfrequency, where the common phase drift is due to different RF clocksand phase noise between the maser AP 310 and the slave AP 320. Assumingthat for tone i,

h_data(i)=h_ndp(i)e ^(j(Øi+θ)) +N _(i)  Eq. (1)

where N_(i) is a noise component for the tone i, Ø corresponds to thetiming error, θ corresponds to the common phase drift. If h_data(i) andh_ndp(i) are known, a phase difference h_diff(i) between h_data andh_ndp(i) can be found as:

$\begin{matrix}{{{h\_ diff}(i)} = {{{h\_ data}*\frac{{conj}({h\_ ndp})}{{h\_ ndp}}} = {{{{{h\_ ndp}(i)}}e^{j{({{\varphi \; i} + \theta})}}} + N_{i}^{\prime}}}} & {{Eq}.\mspace{14mu} (2)}\end{matrix}$

where N′_(i) is a modified noise component for the tone i. Note that thechannel phase comes out, and a real positive number multiplied by aphasor is obtained. In one aspect, this represents that the channelimpact manifests itself through amplitude variations over frequency.With MIMO configuration this metric is summed up across the receiveantennas, in some embodiments. In one aspect, to determine the timingerror, the following equation can be used:

$\begin{matrix}{{{angle}{\sum\limits_{i}{{h\_ diff}\left( {i + K} \right){h\_ diff}(i)^{*}}}} = {{{angle}{\sum\limits_{i}{{{{h\_ ndp}\left( {i + K} \right)}}{{{h\_ ndp}(i)}}e^{j{({\varphi \; K})}}}}} + {\overset{\sim}{N}}_{i}}} & {{Eq}.\mspace{14mu} (3)}\end{matrix}$

where {circumflex over (N)}_(i) is a noise component for the tone i.Because the channel phase is not present in the timing metric shown inEq. (3), increasing K is allowed, as this is the only contributor to theangle, in some embodiments. In one aspect, to determine the common phasedrift, following equation can be used:

angle(Σ_(i)(h_diff(i)*h_diff(−i)))/2  Eq. (4).

According to the timing error obtained through Eq. (3) and the commonphase drift obtained through Eq. (4), synchronization information can bedetermined. Moreover, according to the synchronization information, themaster AP 310 and the slave APs 320A, 320B may perform a jointtransmission.

FIG. 3B is a flow chart illustrating an example process 350 ofestablishing an example multiple-input multiple-output (MIMO)communication, according to some embodiments. In some embodiments, theprocess 350 is performed by the master AP 310, or by the master AP 310and the slave APs 320A, 320B. In other embodiments, the process 350 isperformed by other entities. In other embodiments, the process 350includes more, fewer, or different steps than shown in FIG. 3B.

In one approach, the master AP 310 and the slave APs 320A, 320B perform360 sounding. For example, a soundings controller 222 of the master AP310 may initiate a sounding to obtain or gather channel estimationinformation. The soundings controller 222 of the master AP 310 may causethe transceiver 228 of the master AP 310 to transmit through a wirelessmedium, to the slave APs 320A, 320B, a frame or a portion of the framethat allows the slave APs 320A, 320B to obtain synchronizationinformation for a joint transmission to perform sounding. For exampleand in some embodiments, the soundings controller 222 of the master AP310 causes the transceiver 228 of the master AP 310 to transmit a slavetrigger frame or a preamble of a null data packet frame that allowssoundings controllers 222 of the slave APs 320A, 320B to determine orestimate a carrier frequency offset, a sampling frequency offset, aphase offset, a timing offset, a first reference value for a commonphase offset, and/or a second reference value for a timing offset withrespect to the master AP 310. In one aspect, it may be difficult toascertain the absolute phase offset and timing offset of a slave AP withrespect to the master AP. In some embodiments, the slave APs 320A, 320Bcan estimate the change in phase/timing offset between sounding andsteering (via the preceding trigger frames), according to Eq. (1)-(4)above. The soundings controller 222 of the master AP 310 causes thetransceiver 228 of the master AP 310 to transmit a portion of a frame(e.g., NDP frame) to the station devices 330A-330E to cause the stationdevices 330A-330E to perform channel estimations. The soundingscontrollers 222 of the slave APs 320A, 320B may cause transceivers 228of the slave APs 320A, 320B to transmit the portion of the frame to thestation devices 330A-330E according to the synchronization information,while the master AP 310 transmits the portion of the frame (e.g., NDPframe). The joint transmission by the master AP 310 and the slave APs320A, 320B allows the station devices 330A-330E to successfully receiveand decode the portion of the frame. In one approach, the stationdevices 330A-330E may measure or determine signal strengths and/orsignal qualities of the joint transmission for sounding, and generatechannel estimation information indicating the measured signal strengthsor signal qualities. Detailed descriptions on example frame formats andoperations of sounding are provided below with respect to FIGS. 5-9.

In one approach, the master AP 310 generates 365 steering information inresponse to the channel estimation information. In some embodiments, theintegration module 221 of the master AP 310 receives and integrateschannel estimation information from different station devices 330A-330B,and determines or generates steering information to perform steeringaccording to the integrated channel estimation information. In oneaspect, the steering information indicates, for each AP, configurationof the transceiver 228 for steering. For example, the steeringinformation indicates timing, power, gain, channel bandwidth, frequencyfor transmission through one or more antennas, steering vector, orindicates how to radiate energy in a particular direction,frequency-dependency within a channel, signal-to-noise ratio (SNR),and/or appropriate data rate adjustments for joint transmission withother access points. The integration module 221 of the master AP 310generates the steering information according to the channel estimationinformation, that allows joint transmission by the access points 310,320A, 320B. In one aspect, the master AP 310 generates the steeringinformation and shares the steering information with the slave APs 320A,320B. In some embodiments, each slave AP 320 generates the steeringinformation for itself, provided that the same channel information isavailable at all APs.

In one approach, the master AP 310 performs 370 steering according tothe steering information. For example, a steering controller 225 of themaster AP 310 may initiate a steering. The steering controller 225 ofthe master AP 310 may cause the transceiver 228 of the master AP 310 totransmit, to the slave APs 320A, 320B, a frame that allows the slave APs320A, 320B to obtain synchronization information for a jointtransmission to perform steering. For example, the steering controller225 of the master AP 310 causes the transceiver 228 of the master AP 310to transmit a slave trigger frame or a preamble of a steered frame thatallows steering controllers 225 of the slave APs 320A, 320B to determineor estimate a carrier frequency offset, a sampling frequency offset, aphase offset, a timing offset, a first reference value for a commonphase offset, and/or a second reference value for a timing offset withrespect to the master AP 310. In one aspect, it may be difficult toascertain the absolute phase offset and timing offset of a slave AP withrespect to the master AP. In some embodiments, the slave APs 320A, 320Bcan estimate the change in phase/timing offset between sounding andsteering (via the preceding trigger frames), according to Eq. (1)-(4)above. The steering controller 225 of the master AP 310 causes thetransceiver 228 of the master AP 310 to transmit a portion of a steeredframe to the station devices 330A-330E. The portion of the steered framemay include content data (e.g., text, image, video, or any data). Thesteering controller 225 of the slave APs 320A, 320B may causetransceivers 228 of the slave APs 320A, 320B to transmit the portion ofthe steered frame to the station devices 330A-330E according to thesynchronization information, while the master AP 310 transmits theportion of the steered frame. The joint transmission by the master AP310 and the slave APs 320A, 320B allows the station devices 330A-330E tosuccessfully receive and decode the portion of the steered frame andextract content data from the steered frame. Detailed descriptions onexample frame formats and operations of steering are provided below withrespect to FIGS. 10-13.

FIG. 4 illustrates an example timing diagram of a joint transmission bya master AP and a slave AP to a station device. In some embodiments, thesoundings controller 222 of the master AP causes or configures itstransceiver 228 to communicate with the slave AP and the station device.In some embodiments, the soundings controller 222 of the slave AP causesor configures its transceiver 228 to communicate with the master AP andthe station device. In other embodiments, a different number of masterAPs, slave APs, or station devices may be included for the jointtransmission.

In some embodiments, the soundings controller 222 of the master APcauses or configures the transceiver 228 of the master AP to transmit410 a slave trigger frame to the slave AP. In some embodiments, a slavetrigger frame is a frame transmitted by a master AP for triggering someor all slave APs to transmit a subsequent frame jointly. In one aspect,the slave trigger frame is similar to a trigger frame for HE TB PPDU in802.11ax. In one aspect, the slave trigger frame may be replaced by apreamble of a subsequent frame. In some embodiments, the slave triggerframe indicates which slave APs to participate in the subsequent jointtransmission, which could change dynamically from one joint TX to thenext. In some embodiments, the slave trigger frame allows slave APs toestimate their CFO/SFO, phase-offset (PO), timing-offset (TO), areference value for PO, and/or a reference value for TO relative tomaster AP, by means of long training fields (LTFs) included in theframe. In order to improve the accuracy of these estimates, the numberof LTFs may be larger than the number sufficient for the channelestimation. In some embodiments, these LTFs may be locatednon-contiguously in the frame, e.g., in mid-ambles or in a post-amble.In some embodiments, the slave trigger frame contains PHY/MAC levelinformation to construct and transmit subsequent frame jointly, e.g.,and may contain frame-type/purpose, rate, receiver MAC address(es),payload, etc. In some embodiments, the slave trigger frame contains datafor slave AP to be used in future joint AP transmissions either in asubsequent packet or within the same packet with some delay. In someembodiments, the soundings controller 222 of the slave AP causes orconfigures its transceiver 228 to receive the slave trigger frame, anddecodes the slave trigger frame to generate 415 synchronizationinformation by estimating CFO, SFO, PO, TO, a reference value for PO,and/or a reference value for TO with respect to the master AP. In oneaspect, the slave trigger frame may not be successfully heard or decoded418 by station devices.

In some embodiments, the soundings controller 222 of the master APcauses its transceiver 228 to transmit 420 a joint frame to a stationdevice, for example, after a short inter-frame spacing (SIFS) followingthe end of the slave trigger frame. During the short inter-framespacing, the master AP and the slave AP may not transmit. In one aspect,the soundings controller 222 of the slave AP causes its transceiver 228to transmit 422 the joint frame to the station device, while the masterAP transmits 420 the joint frame. The soundings controller 222 of theslave AP may correct for their CFO, SFO, PO, TO, a reference value forPO, and/or a reference value for TO with respect to the master AP andjointly transmit with the master AP. In one aspect, CSI-dependentsteering vectors may not be applied, such that the joint transmission bythe master AP and the slave AP may not be beamformed. In someembodiments, the joint transmission may be broadcast/multicast, orunicast in the absence of CSI. In one aspect, some or all APs transmitidentical preambles and payloads in the joint frame, encoded at the samePHY rate. In some embodiments, the soundings controllers 222 of APsmodulate signals for transmission with a different set of cyclic delays,to avoid unintentional beamforming, such as in 802.11n/ac/ax single-usertransmissions.

In some embodiments, the station device receives the joint transmissionby the master AP and the slave AP, and processes or decodes 428 thejoint transmission. For example, the station device measures ordetermines signal strengths or signal qualities of the jointtransmission, and generates channel estimation information indicatingthe measured signal strengths or signal qualities. In some embodiments,the station device transmits or provides the channel estimationinformation to the master AP, the slave AP, or any AP.

FIG. 5 illustrates an example timing diagram of a sounding sequence,according to some embodiments. In some embodiments, the master AP andthe slave AP perform a joint transmission for sounding. In otherembodiments, a different number of master APs, slave APs, or stationdevices may be included for the joint transmission.

In some embodiments, the soundings controller 222 of the master APconfigures or causes its transceiver to transmit 510 a slave triggerframe to the slave AP. In one aspect, the slave trigger frame alerts theslave AP to commence the sounding sequence. In one aspect, the slavetrigger frame contains information to be transmitted in subsequent NDPannouncement frame jointly with the slave AP jointly. In one aspect, theslave trigger frame indicates contents of preamble 2 and/or a number ofLTFs in a NDP frame. The slave trigger frame may be decoded by some orall slave APs. In some embodiments, the soundings controller 222 of theslave AP estimates 515 synchronization information for an NDPannouncement frame. For example, the soundings controller 222 of theslave AP measures or estimates CFO, SFO, PO, TO, a reference value forPO, and/or a reference value for TO relative to master AP, andsynchronizes with the master AP for the subsequent NDP announcementframe according to the CFO, SFO, PO, TO, a reference value for PO,and/or a reference value for TO. In some embodiments, the station device518 may or may not decode the slave trigger frame.

In some embodiments, the soundings controller 222 of the master APcauses its transceiver 228 to transmit 520 a NDP announcement frame to astation device, for example, after a SIFS following the end of the slavetrigger frame. During the short inter-frame spacing, the master AP andthe slave AP may not transmit. In some embodiments, an NDP announcementframe contains information on which station devices are expected tofeedback channel information, and a type of channel information. In oneaspect, the soundings controller 222 of the slave AP causes itstransceiver 228 to transmit 522 the NDP announcement frame to thestation device, while the master AP transmits 520 the NDP announcementframe. In one aspect, the slave AP synchronizes with the master AP,according to the synchronization information for the joint transmissionof the NDP announcement frame. The station device may receive the jointtransmission of the NDP announcement frame by the master AP and theslave AP, and prepare 528 for subsequent NDP frame, similar to HE/VHTsounding sequence. In some embodiments, the station device checks if thestation device is on a list of devices supposed to receive thesubsequent NDP frame.

In some embodiments, the soundings controller 222 of the master APcauses its transceiver 228 to transmit a NDP, for example, after a SIFSfollowing the end of the NDP announcement frame. During the shortinter-frame spacing, the master AP and the slave AP may not transmit. Insome embodiments, the NDP frame includes a preamble 1, preamble 2, andLTFs. In one aspect, the preamble 1 is transmitted 530 by the master APbut not by the slave AP. In some embodiments, the slave AP receives thepreamble 1 and estimates 535 or updates synchronization information(e.g., CFO, SFO, PO, TO, a reference value for PO, and/or a referencevalue for TO with respect to the master AP) according to the preamble 1.In one aspect, the preamble 1 allows the slave AP sufficient time toswitch from a receive mode to a transmit mode for the joint transmissionof the remaining portion of the NDP frame. In some embodiments, thepreamble 1 indicates contents of a portion of a subsequent frame (e.g.,preamble 2) and/or the number of LTFs. In some embodiments, the stationdevice may not hear/decode 538 the preamble 1, or may choose to ignorethe preamble 1.

In some embodiments, the soundings controller 222 of the master APcauses its transceiver 228 to transmit 540 the preamble 2 subsequent tothe end of the preamble 1. In some embodiments, the soundings controller222 of the slave AP causes its transceiver 228 to transmit 542 thepreamble 2 to the station device according to the synchronizationinformation, while the master AP transmits the preamble 2. In someembodiments, the joint transmission of the preamble 2 by the master APand the slave AP causes or allows the station device to receive, decode,and/or process LTFs, such as a mixed-mode preamble in HE/VHT NDPs.

In some embodiments, the soundings controller 222 of the master APcauses its transceiver 228 to transmit 550 the LTF after the end of thepreamble 2. In some embodiments, the soundings controller 222 of theslave AP configures its transceiver 228 to transmit 552 the LTF to thestation device according to the synchronization information, while themaster AP transmits 550 the LTF. In some embodiments, the jointtransmission of the LTFs by the master AP and the slave AP instructs,causes or allows the station device to measure channels from the masterAP and the slave AP. In one aspect, a total number of LTFs can exceedthe maximum number of sounding dimensions supported by any single AP. Insome embodiments, the master AP and the slave AP(s) independentlymodulate the LTFs with different orthogonal codes (e.g., different rowsof a Hadamard matrix, similar to HE UL MUMIMO transmission), for ease ofseparation at the station device. In some embodiments, the stationdevice detects the NDP frame and generates 548 channel estimationinformation for downlink channels according to the LTFs. In oneapproach, the station device transmits the channel estimationinformation to the master AP and/or the slave AP. The integration module221 of the master AP or the slave AP may integrate channel estimationinformation from station devices, and compute steering vectors for jointMU-MIMO transmission.

FIG. 6 illustrates an example timing diagram of another soundingsequence, according to some embodiments. In some embodiments, the masterAP and the slave AP perform a joint transmission for sounding. In otherembodiments, a different number of master APs, slave APs, or stationdevices may be included for the joint transmission.

In some embodiments, the soundings controller 222 of the master APconfigures or causes its transceiver to transmit 610 a slave triggerframe 1 to the slave AP. In one aspect, the slave trigger 1 alerts theslave AP to commence the sounding sequence. In one aspect, the slavetrigger frame 1 contains information to be transmitted in a subsequentNDP announcement frame with the slave AP jointly. In one aspect, theslave trigger frame 1 indicates contents of NDP frame and/or a number ofLTFs in the NDP frame. The slave trigger frame 1 may be decoded by someor all slave APs. In some embodiments, the soundings controller 222 ofthe slave AP estimates 615 synchronization information for an NDPannouncement frame. For example, the soundings controller 222 of theslave AP measures or estimates CFO, SFO, PO, TO, a reference value forPO, and/or a reference value for TO relative to master AP, andsynchronizes with the master AP for the subsequent NDP announcementframe according to the CFO, SFO, PO, TO, a reference value for PO,and/or a reference value for TO. In some embodiments, the station device618 may or may not decode the slave trigger frame 1.

In some embodiments, the soundings controller 222 of the master APcauses its transceiver 228 to transmit 620 a NDP announcement frame to astation device, for example, after a SIFS following the end of the slavetrigger frame 1. During the short inter-frame spacing, the master AP andthe slave AP may not transmit. In some embodiments, an NDP announcementframe contains information on which station devices are expected tofeedback channel information, and a type of channel information. In oneaspect, the soundings controller 222 of the slave AP causes itstransceiver 228 to transmit 622 the NDP announcement frame to thestation device, while the master AP transmits 620 the NDP announcementframe. In one aspect, the slave AP synchronizes with the master AP,according to the synchronization information for the joint transmissionof the NDP announcement frame. The station device may receive the jointtransmission of the NDP announcement frame by the master AP and theslave AP, and prepare 628 for subsequent NDP frame, similar to HE/VHTsounding sequence. In some embodiments, the station device checks if thestation device is on a list of devices supposed to receive thesubsequent NDP frame.

In some embodiments, the soundings controller 222 of the master APcauses its transceiver 228 to transmit 630 a slave trigger frame 2, forexample, after a SIFS following the end of the NDP announcement frame.During the short inter-frame spacing, the master AP and the slave AP maynot transmit. In one aspect, the slave trigger frame 2 is transmitted630 by the master AP but not by the slave AP. In some embodiments, thesoundings controller 222 of the slave AP configures the transceiver 228of the slave AP to receive the slave trigger frame 2, and estimates 635or updates synchronization information (e.g., CFO, SFO, PO, TO, areference value for PO, and/or a reference value for TO with respect tothe master AP) according to the slave trigger frame 2. In one aspect,the slave trigger frame 2 allows the slave AP sufficient time to switchfrom a receive mode to a transmit mode for the joint transmission of theNDP frame. In some embodiments, the slave trigger frame 2 indicatescontents of the NDP frame and/or the number of LTFs in the NDP frame. Insome embodiments, the station device may not hear/decode 638 the slavetrigger frame 2, or may choose to ignore slave trigger frame 2.

In some embodiments, the soundings controller 222 of the master APcauses its transceiver 228 to transmit 640 the NDP, for example, after aSIFS following the end of the slave trigger frame 2. During the shortinter-frame spacing, the master AP and the slave AP may not transmit. Insome embodiments, the slave AP transmits 642 the NDP frame to thestation device according to the synchronization information, while themaster AP transmits 640 the NDP frame. In some embodiments, the jointtransmission of the NDP frame by the master AP and the slave AP causesor allows the station device to receive, decode, and/or process LTFs,such as a mixed-mode preamble in HE/VHT NDPs. In some embodiments, thejoint transmission of the NDP frame by the master AP and the slave APinstructs, causes or allows the station device to measure channels fromthe master AP and the slave AP. In one aspect, a total number of LTFs inthe NDP frame can exceed the maximum number of sounding dimensionssupported by any single AP. In some embodiments, the master AP and theslave AP(s) independently modulate the LTFs in the NDP frame withdifferent orthogonal codes (e.g., different rows of a Hadamard matrix,similar to HE UL MUMIMO transmission), for ease of separation at thestation device. In some embodiments, the station device detects the NDPframe and generates 648 channel estimation information for downlinkchannels according to the LTFs in the NDP frame. In one approach, thestation device transmits the channel estimation information to themaster AP and/or the slave AP. The integration module 221 of the masterAP or the slave AP may integrate channel estimation information fromstation devices, and compute steering vectors for joint MU-MIMOtransmission.

FIG. 7 illustrates an example timing diagram of another soundingsequence, according to some embodiments. In some embodiments, the masterAP and the slave AP perform a joint transmission for sounding. In otherembodiments, a different number of master APs, slave APs, or stationdevices may be included for the joint transmission.

In some embodiments, the soundings controller 222 of the master APcauses its transceiver 228 to transmit 710 a NDP announcement frame tothe slave AP and the station device. In some embodiments, an NDPannouncement frame notifies or indicates a joint transmission of NDPframe. The NDP announcement frame may contain information for stationdevices to process NDP and construct CSI feedback. In some embodiments,the NDP announcement frame serves as a slave trigger frame in FIG. 5 orFIG. 6. The NDP announcement frame may indicate participating slave APsfor the joint transmission of NDP frame. The NDP announcement frame mayallow slave APs to estimate CFO, SFO, PO, TO, a reference value for PO,and/or a reference value for TO relative to the master AP. Moreover, theNDP announcement frame may contain information on how to construct theNDP frame (e.g., a number of LTFs, etc.). In some embodiments, thesoundings controller 222 of the slave AP configures the transceiver 228of the slave AP to receive the NDP announcement frame, and estimates 715or updates synchronization information (e.g., CFO, SFO, PO, TO, areference value for PO, and/or a reference value for TO with respect tothe master AP) according to the NDP announcement frame. In someembodiments, the NDP announcement frame indicates contents of the NDPframe and/or the number of LTFs in the NDP frame. In some embodiments,the station device receives and decodes the NDP announcement frame bythe master AP, and obtains 718 information to receive or process the NDPand construct CSI feedback.

In some embodiments, the soundings controller 222 of the master APcauses its transceiver 228 to transmit 720 the NDP, for example, after aSIFS following the end of the NDP announcement frame. During the shortinter-frame spacing, the master AP and the slave AP may not transmit. Insome embodiments, the soundings controller 222 of the slave APconfigures or causes the transceiver 228 of the slave AP to transmit 722the NDP frame to the station device according to the synchronizationinformation, while the master AP transmits 720 the NDP frame. In someembodiments, the joint transmission of the NDP frame by the master APand the slave AP causes or allows the station device to receive, decode,and/or process LTFs in the NDP frame. In some embodiments, the jointtransmission of the NDP frame by the master AP and the slave APinstructs, causes or allows the station device to measure channels fromthe master AP and the slave AP. In one aspect, a total number of LTFs inthe NDP frame can exceed the maximum number of sounding dimensionssupported by any single AP. In some embodiments, the master AP and theslave AP(s) independently modulate the LTFs in the NDP frame withdifferent orthogonal codes (e.g., different rows of a Hadamard matrix,such as in HE UL MUMIMO transmission), for ease of separation at thestation device. In some embodiments, the station device detects the NDPframe and generates 728 channel estimation information for downlinkchannels according to the LTFs in the NDP frame. In one approach, thestation device transmits the channel estimation information to themaster AP and/or the slave AP. The integration module 221 of the masterAP or the slave AP may integrate channel estimation information fromstation devices, and compute steering vectors for joint MU-MIMOtransmission.

FIG. 8 illustrates an example timing diagram of another soundingsequence, according to some embodiments. In some embodiments, the masterAP and the slave AP perform a joint transmission for sounding. In otherembodiments, a different number of master APs, slave APs, or stationdevices may be included for the joint transmission.

In some embodiments, the soundings controller 222 of the master APcauses its transceiver 228 to transmit 810 a first portion of a frame(e.g., NDP frame) to the slave AP and the station device. In someembodiments, the first portion of the frame includes a preamble and apayload. In some embodiments, the preamble of the first portion of theframe could indicate special frame type to distinguish from other typesof sounding sequences. In some embodiments, the first portion of theframe indicates a number of trailing LTFs, and positions of trailing STFand LTFs. In some embodiments, the first portion of the frame identifiesparticipating stations and notifies sounding parameters, CSI expected,etc. In some embodiments, the first portion of the frame identifiesparticipating slave APs, and allows estimation of CFO, SFO, PO, TO, areference value for PO, and/or a reference value for TO with respect tothe master AP. In some embodiments, the first portion of the frameallows slave APs to transition from a receive mode to a transmit mode.In some embodiments, the first portion of the frame serves as a slavetrigger frame in FIG. 5 or FIG. 6. In some embodiments, the soundingscontroller 222 of the slave AP configures the transceiver 228 of theslave AP to receive the first portion of the frame, and estimates 815 orupdates synchronization information (e.g., CFO, SFO, PO, TO, a referencevalue for PO, and/or a reference value for TO with respect to the masterAP) according to the first portion of the frame. In some embodiments,the station device receives and decodes 818 the first portion of theframe transmitted by the master AP, and obtains information for CSIfeedback estimation.

In some embodiments, the soundings controller 222 of the master APcauses its transceiver 228 to transmit 820 a second portion of theframe, subsequent to the first portion of the frame. In someembodiments, the second portion of the frame includes a short trainingfield (STF). In some embodiments, the soundings controller 222 of theslave AP configures or causes the transceiver 228 of the slave AP totransmit 822 the second portion of the frame to the station deviceaccording to the synchronization information, while the master APtransmits the second portion of the frame. In some embodiments, thejoint transmission of the second portion of the frame by the master APand the slave AP causes or allows the station device to receive thesecond portion of the frame and perform 828 automatic gain control (AGC)or adjust gain settings.

In some embodiments, the soundings controller 222 of the master APcauses its transceiver 228 to transmit 830 a third portion of the frame,subsequent to the second portion of the frame. In some embodiments, thethird portion of the frame includes a long training field (LTF). In someembodiments, the soundings controller 222 of the slave AP configures orcauses the transceiver 228 of the slave AP to transmit 832 the thirdportion of the frame to the station device according to thesynchronization information, while the master AP transmits 830 the thirdportion of the frame. In some embodiments, the joint transmission of thethird portion of the frame by the master AP and the slave AP causes orallows the station device to receive, decode, and/or process LTFs in thethird portion of the frame. In some embodiments, the joint transmissionof the third portion of the frame by the master AP and the slave APinstructs, causes or allows the station device to measure channels fromthe master AP and the slave AP. In one aspect, a total number of LTFs inthe third portion of the frame can exceed the maximum number of soundingdimensions supported by any single AP. In some embodiments, the masterAP and the slave AP(s) independently modulate the LTFs in the thirdportion of the frame with different orthogonal codes (e.g., differentrows of a Hadamard matrix, similar to HE UL MUMIMO transmission), forease of separation at the station device. In some embodiments, thestation device detects the third portion of the frame and generates 838channel estimation information for downlink channels according to theLTFs in the third portion of the frame. In one approach, the stationdevice transmits the channel estimation information to the master APand/or the slave AP. The integration module 221 of the master AP or theslave AP may integrate channel estimation information from stationdevices, and compute steering vectors for joint MU-MIMO transmission.

FIG. 9 illustrates an example timing diagram of another soundingsequence, according to some embodiments. In some embodiments, the masterAP and the slave AP perform a joint transmission for sounding. In otherembodiments, a different number of master APs, slave APs, or stationdevices may be included for the joint transmission.

In some embodiments, the soundings controller 222 of the master APconfigures or causes its transceiver to transmit 910 a slave triggerframe to the slave AP. In one aspect, the slave trigger frame alerts theslave AP to commence the sounding sequence. In one aspect, the slavetrigger frame contains information to be transmitted in subsequent NDPannouncement frame with the slave AP jointly. In one aspect, the slavetrigger frame indicates contents of NDP frame and/or a number of LTFs inthe NDP frame. The slave trigger frame may be decoded by some or allslave APs. In some embodiments, the soundings controller 222 of theslave AP estimates 915 synchronization information (e.g., CFO, SFO, PO,TO, a reference value for PO, and/or a reference value for TO withrespect to the master AP) for an NDP announcement frame. For example,the soundings controller 222 of the slave AP measures or estimates CFO,SFO, PO, TO, a reference value for PO, and/or a reference value for TOrelative to master AP, and synchronizes with the master AP for thesubsequent NDP announcement frame according to the CFO, SFO, PO, TO, areference value for PO, and/or a reference value for TO. In someembodiments, the station device 918 may or may not decode the slavetrigger frame.

In some embodiments, the soundings controller 222 of the master APcauses its transceiver 228 to transmit 920 a NDP announcement frame to astation device, for example, after a SIFS following the end of the slavetrigger frame. During the short inter-frame spacing, the master AP andthe slave AP may not transmit. In some embodiments, an NDP announcementframe contains information on which station devices are expected tofeedback channel information, and a type of channel information. In oneaspect, the soundings controller 222 of the slave AP causes itstransceiver 228 to transmit 922 the NDP announcement frame to thestation device, while the master AP transmits 920 the NDP announcementframe. In one aspect, the slave AP synchronizes with the master AP,according to the synchronization information for the joint transmissionof the NDP announcement frame. The station device may receive the jointtransmission of the NDP announcement frame by the master AP and theslave AP, and prepare 928 for subsequent NDP frame, similar to HE/VHTsounding sequence. In some embodiments, the station device checks if thestation device is on a list of devices supposed to receive thesubsequent NDP frame.

In some embodiments, the soundings controller 222 of the master APcauses its transceiver 228 to transmit 930 the NDP, for example, after aSIFS following the end of the NDP announcement frame. During the shortinter-frame spacing, the master AP and the slave AP may not transmit. Insome embodiments, the slave AP transmits 932 the NDP frame to thestation device according to the synchronization information, while themaster AP transmits 930 the NDP frame. In some embodiments, the jointtransmission of the NDP frame by the master AP and the slave AP causesor allows the station device to receive, decode, and/or process LTFs,such as in a mixed-mode preamble in HE/VHT NDPs. In some embodiments,the joint transmission of the NDP frame by the master AP and the slaveAP instructs, causes or allows the station device to measure channelsfrom the master AP and the slave AP. In one aspect, a total number ofLTFs in the NDP frame can exceed the maximum number of soundingdimensions supported by any single AP. In some embodiments, the masterAP and the slave AP(s) independently modulate the LTFs in the NDP framewith different orthogonal codes (e.g., different rows of a Hadamardmatrix, similar to HE UL MUMIMO transmission), for ease of separation atthe station device. In some embodiments, the station device detects theNDP frame and generates 938 channel estimation information for downlinkchannels according to the LTFs in the NDP frame. In one approach, thestation device transmits the channel estimation information to themaster AP and/or the slave AP. The integration module 221 of the masterAP or the slave AP may integrate channel estimation information fromstation devices, and compute steering vectors for joint MU-MIMOtransmission.

FIG. 10 illustrates an example timing diagram of a steering sequence,according to some embodiments. In some embodiments, the master AP andthe slave AP perform a joint transmission for steering. In otherembodiments, a different number of master APs, slave APs, or stationdevices may be included for the joint transmission.

In some embodiments, the steering controller 225 of the master APconfigures or causes its transceiver to transmit 1010 a slave triggerframe to the slave AP. In one aspect, the slave trigger frame alerts theslave AP to commence the steering sequence. In one aspect, the slavetrigger frame includes a preamble that allows the slave AP to determineor estimate synchronization information. In one aspect, the slavetrigger frame includes a payload indicating how to construct a steeredframe (e.g., targeted station devices for steered frame, PHY/MACtransmission parameters for each station device, which data frames totransmit to each station device, etc.). In one approach, the steeringcontroller 225 of the slave AP computes steering vectors for a jointtransmission of the steered frame to the indicated station devices. Theslave trigger frame may be decoded by some or all slave APs. In someembodiments, the steering controller 225 of the slave AP estimates 1015synchronization information for a steered frame. For example, thesteering controller 225 of the slave AP measures or estimates CFO, SFO,PO, TO relative to master AP, and synchronizes with the master AP forthe subsequent steered frame according to the CFO, SFO, PO, TO. In someembodiments, PO, TO, or a combination of PO and TO is not determinedduring the sounding sequence, and the steering controller 225 of theslave AP may determine, in the step 1015, a change in PO and/or TObetween the sounding sequence and the steering sequence as describedabove with respect to Eq. (1)-Eq. (4). In some embodiments, the stationdevice may or may not decode 1018 the slave trigger frame.

In some embodiments, the steering controller 225 of the master AP causesits transceiver 228 to transmit 1020 a preamble of the steered frame toa station device, for example, after a SIFS following the end of theslave trigger frame. During the short inter-frame spacing, the master APand the slave AP may not transmit. In some embodiments, the steeringcontroller 225 of the slave AP causes its transceiver 228 to transmit1022 the preamble of the steered frame to the station device, while themaster AP transmits 1020 the preamble to the station device. In someembodiments, the station device receives and processes 1028 thepreamble. In one aspect, the station device prepares for a subsequenttransmission of data. In some embodiments, the station device estimatesor determines a field (e.g., signal field) that includes information todecode the payload in the subsequent joint transmission by the master APand the slave AP. In one aspect, the master AP and the slave AP jointlytransmit the steered frame by applying respective steering vectorsindicated in the steering information. In one aspect, the steeringcontroller 225 of the slave AP compensates for CFO, SFO, TO, PO, areference value for PO, and/or a reference value for TO for the jointtransmission of the preamble according to the synchronizationinformation.

In some embodiments, the steering controller 225 of the master AP causesits transceiver 228 to transmit 1030 content data of the steered frameto the station device, for example, after the preamble. In someembodiments, the steering controller 225 of the slave AP causes itstransceiver 228 to transmit 1032 the content data of the steered frameto the station device, while the master AP transmits 1030 the contentdata to the station device. In some embodiments, the station devicereceives and decodes 1038 the joint transmission of the content data.

In some embodiments, the steering controller 225 of the master AP causesits transceiver to transmit 1040 a mid-amble of the steered frame, forexample, after transmitting 1030 content data. In some embodiments, thesteering controller 225 of the slave AP estimates 1045 or adjustssynchronization information for the remaining portion of the steeredframe. For example, the steering controller 225 of the slave AP measuresor adjusts CFO, SFO, PO, TO, a reference value for PO, and/or areference value for TO relative to the master AP. In some embodiments,PO, TO, or a combination of PO and TO is not determined during thesounding sequence, and the steering controller 225 of the slave AP maydetermine, in the step 1045, a change in PO and/or TO between thesounding sequence and the steering sequence as described above withrespect to Eq. (1)-Eq. (4). In one aspect, the slave AP may nottransmit, while the master AP transmits 1040 the mid-amble. In someembodiments, the station device may or may not decode 1048 the slavetrigger frame. In some embodiments, the slave AP updates thesynchronization information for the rest of the frame by using thecontent of the mid-amble. In some embodiments, the mid-amble allowssufficient time for the slave APs to switch from a transmit mode to areceive mode before updating the synchronization information and switchback to the transmit mode after updating the synchronizationinformation.

In some embodiments, the steering controller 225 of the master AP causesits transceiver 228 to transmit 1050 content data of the steered frameto the station device, for example, after the mid-amble. In someembodiments, the steering controller 225 of the slave AP causes itstransceiver 228 to transmit 1052 the content data of the steered frameto the station device, while the master AP transmits 1050 the contentdata to the station device. In some embodiments, the station devicereceives and decodes 1058 the joint transmission of the content data. Insome embodiments, during the transmission 1052, the slave APs use theupdated synchronization information obtained from the mid-amble tojointly transmit along with the master AP.

FIG. 11 illustrates an example timing diagram of a steering sequence,according to some embodiments. In some embodiments, the master AP andthe slave AP perform a joint transmission for steering. In otherembodiments, a different number of master APs, slave APs, or stationdevices may be included for the joint transmission.

In some embodiments, the steering controller 225 of the master APconfigures or causes its transceiver to transmit 1110 steeringinformation of a frame (e.g., a slave trigger frame) to the slave AP. Inone aspect, the steering information of the frame alerts the slave AP tocommence the steering sequence. In one aspect, the steering informationof the frame includes a preamble and a payload. The preamble of thesteering information may indicate a frame type to distinguish from othertypes of frames. In some embodiments, the steering information of theframe provides information for the steering controller 225 of the slaveAP to decode a portion of the frame to extract content data. In oneaspect, the steering information of the frame indicates a presence ofany mid-amble in the frame. In one aspect, the steering information ofthe frame serves as the slave trigger frame of FIG. 10. In one approach,the steering controller 225 of the slave AP computes steering vectorsfor a joint transmission of the frame to the indicated station devices.The steering information of the frame may be decoded by some or allslave APs. In some embodiments, the steering controller 225 of the slaveAP estimates 1115 synchronization information for the frame (e.g.,steered frame). For example, the steering controller 225 of the slave APmeasures or estimates CFO, SFO, PO, TO, a reference value for PO, and/ora reference value for TO relative to master AP, and synchronizes withthe master AP for the subsequent steered frame according to the CFO,SFO, PO, TO, a reference value for PO, and/or a reference value for TO.In some embodiments, PO, TO, or a combination of PO and TO is notdetermined during the sounding sequence, and the steering controller 225of the slave AP may determine, in the step 1115, a change in PO and/orTO between the sounding sequence and the steering sequence as describedabove with respect to Eq. (1)-Eq. (4). In one aspect, the station devicereceives the joint transmission of the steering information of the frameand prepares 1118 to decode the remaining portion of the steered frame.In some embodiments, the station device estimates or determines a field(e.g., signal field) that includes information to decode the payload inthe subsequent joint transmission by the master AP and the slave AP.

In some embodiments, the steering controller 225 of the master AP causesits transceiver 228 to transmit 1120 a STF of the frame (e.g., steeredframe) to a station device, for example, after transmitting the steeringinformation. In some embodiments, the steering controller 225 of theslave AP causes its transceiver 228 to transmit 1122 the STF of theframe to the station device, while the master AP transmits 1120 the STFto the station device. In some embodiments, the master AP and the slaveAP jointly transmit the steered frame by applying respective steeringvectors indicated in the steering information. In some embodiments, theSTF transmitted by the master AP and the slave AP allows the stationdevice to perform 1128 automatic gain control (AGC) or adjust gainsettings.

In some embodiments, the steering controller 225 of the master AP causesits transceiver 228 to transmit 1130 a LTF of the frame (e.g., steeredframe) to a station device, for example, after transmitting the STF. Insome embodiments, the steering controller 225 of the slave AP causes itstransceiver 228 to transmit 1132 the LTF of the frame to the stationdevice, while the master AP transmits 1130 the LTF to the stationdevice. In some embodiments, the master AP and the slave AP jointlytransmit the steered frame by applying respective steering vectorsindicated in the steering information. In some embodiments, the LTFtransmitted by the master AP and the slave AP allows the station deviceto estimate 1138 channel information. According to the channelestimation, the station device may decode data from the jointtransmission by the master AP and the slave AP.

In some embodiments, the steering controller 225 of the master AP causesits transceiver 228 to transmit 1140 content data of the steered frameto the station device, for example, after the preamble. In someembodiments, the steering controller 225 of the slave AP causes itstransceiver 228 to transmit 1142 the content data of the steered frameto the station device, while the master AP transmits 1140 the contentdata to the station device. In some embodiments, the station devicereceives and decodes 1148 the joint transmission of the content data.

In some embodiments, the steering controller 225 of the master AP causesits transceiver to transmit 1150 a mid-amble of the steered frame, forexample, after transmitting 1140 content data. In some embodiments, thesteering controller 225 of the slave AP estimates 1155 or adjustssynchronization information for the remaining portion of the steeredframe. For example, the steering controller 225 of the slave AP measuresor adjusts CFO, SFO, PO, TO, a reference value for PO, and/or areference value for TO relative to the master AP. In some embodiments,PO, TO, or a combination of PO and TO is not determined during thesounding sequence, and the steering controller 225 of the slave AP maydetermine or update, in the step 1155, a change in PO and/or TO betweenthe sounding sequence and the steering sequence as described above withrespect to Eq. (1)-Eq. (4). In one aspect, the slave AP may nottransmit, while the master AP transmits 1150 the mid-amble. In someembodiments, the station device may or may not decode the slave triggerframe. In some embodiments, the slave AP updates the synchronizationinformation for one or more subsequent frames by using the content ofthe mid-amble. In some embodiments, the mid-amble allows sufficient timefor the slave APs to switch from a transmit mode to a receive modebefore updating the synchronization information and switch back to thetransmit mode after updating the synchronization information.

In some embodiments, the steering controller 225 of the master AP causesits transceiver 228 to transmit 1160 content data of the steered frameto the station device, for example, after the mid-amble. In someembodiments, the steering controller 225 of the slave AP causes itstransceiver 228 to transmit 1162 the content data of the steered frameto the station device, while the master AP transmits 1160 the contentdata to the station device. In some embodiments, the station devicereceives and decodes 1168 the joint transmission of the content data. Insome embodiments, during the transmission 1162, the slave APs use theupdated synchronization information obtained from the mid-amble tojointly transmit along with the master AP.

FIG. 12 illustrates an example timing diagram of a steering sequence,according to some embodiments. In some embodiments, the master AP andthe slave AP perform a joint transmission for steering. In otherembodiments, a different number of master APs, slave APs, or stationdevices may be included for the joint transmission.

In some embodiments, the steering controller 225 of the master APconfigures or causes its transceiver to transmit 1210 a slave triggerframe to the slave AP. In one aspect, the slave trigger frame alerts theslave AP to commence the steering sequence. In one aspect, the slavetrigger frame instructs, causes, or allows the slave AP to determine orestimate synchronization information. In one approach, the steeringcontroller 225 of the slave AP computes steering vectors for a jointtransmission of the first steered frame to the indicated stationdevices. The slave trigger frame may be decoded by some or all slaveAPs. In some embodiments, the steering controller 225 of the slave APestimates 1215 synchronization information for the first steered frame.For example, the steering controller 225 of the slave AP measures CFO,SFO, PO, TO, a reference value for PO, and/or a reference value for TOrelative to master AP, and synchronizes with the master AP for the jointtransmission of the first steered frame. In some embodiments, PO, TO, ora combination of PO and TO is not determined during the soundingsequence, and the steering controller 225 of the slave AP may determine,in the step 1215, a change in PO and/or TO between the sounding sequenceand the steering sequence as described above with respect to Eq. (1)-Eq.(4). In some embodiments, the station device may or may not decode theslave trigger frame.

In some embodiments, the steering controller 225 of the master AP causesits transceiver 228 to transmit 1220 the first steered frame includingcontent data to the station device, for example, after an SIFS (e.g., 16μs) or less following the end of the slave trigger frame. In someembodiments, the master AP and the slave AP may not transmit during theIFS. In some embodiments, the steering controller 225 of the slave APcauses its transceiver 228 to transmit 1222 the first steered frame tothe station device, while the master AP transmits 1220 the first steeredframe to the station device. In some embodiments, the master AP and theslave AP jointly transmit the first steered frame by applying respectivesteering vectors indicated in the steering information. In someembodiments, the station device receives and decodes 1228 the jointtransmission of the content data.

In some embodiments, the steering controller 225 of the master APconfigures or causes its transceiver to transmit 1230 a slave NDP frameto the slave AP, for example, after an inter-frame spacing following theend of the first steered frame. In some embodiments, the IFS may be SIFS(e.g., 16 μs) or less. In one aspect, the slave NDP frame includes apreamble and extra LTFs, with no content data. In one aspect, the slaveNDP frame serves as the mid-amble of FIG. 10 or FIG. 11. In one aspect,the slave trigger frame instructs, causes, or allows the slave AP todetermine or estimate synchronization information. In some embodiments,the steering controller 225 of the slave AP estimates 1235synchronization information for a second steered frame. For example, thesteering controller 225 of the slave AP measures or estimates CFO, SFO,PO, TO, a reference value for PO, and/or a reference value for TOrelative to master AP, and synchronizes with the master AP for thesecond steered frame according to the CFO, SFO, PO, TO, a referencevalue for PO, and/or a reference value for TO. In some embodiments, PO,TO, or a combination of PO and TO is not determined during the soundingsequence, and the steering controller 225 of the slave AP may determineor update, in the step 1235, a change in PO and/or TO between thesounding sequence and the steering sequence as described above withrespect to Eq. (1)-Eq. (4). In some embodiments, the station device mayor may not decode the slave trigger frame.

In some embodiments, the steering controller 225 of the master AP causesits transceiver 228 to transmit 1240 the second steered frame includingcontent data to the station device, for example, after an IFS followingthe end of the slave NDP frame. In some embodiments, the IFS may be SIFS(e.g., 16 μs) or less. In some embodiments, the master AP and the slaveAP may not transmit during the IFS. In some embodiments, the steeringcontroller 225 of the slave AP causes its transceiver 228 to transmit1242 the second steered frame to the station device, while the master APtransmits 1240 the second steered frame to the station device. In someembodiments, the slave AP uses the estimated or updated synchronizationinformation obtained in the step 1235 for the transmission 1242. In someembodiments, the master AP and the slave AP jointly transmit the secondsteered frame by applying respective steering vectors indicated in thesteering information. In some embodiments, the station device receivesand decodes 1248 the joint transmission of the content data.

In one aspect, transmission through the first steered frame and thesecond steered frame with the NDP frame in between can protect againstor correct for any phase drift (e.g., drift from CFO). In one aspect, alarger time-separation between steered frames as shown in FIG. 12 allowsthe slave APs to compare LTFs across multiple slave NDP frames and/orslave trigger frames to improve CFO estimation. In one aspect, IFS amongdifferent frames may vary or change. In some embodiments, time allocatedto the first steered frame and the second steered frame may bedifferent. For example, time allocated for the first steered frame maybe shorter than time allocated for the second steered frame, because theCFO estimation can be more accurate after the estimation based on theslave NDP.

FIG. 13 illustrates an example timing diagram of a steering sequence,according to some embodiments. In some embodiments, the master AP andthe slave AP perform a joint transmission for steering. In otherembodiments, a different number of master APs, slave APs, or stationdevices may be included for the joint transmission.

In some embodiments, the steering controller 225 of the master APconfigures or causes its transceiver to transmit 1310 a slave triggerframe to the slave AP. In one aspect, the slave trigger frame alerts theslave AP to commence the steering sequence. In one aspect, the slavetrigger frame instructs, causes, or allows the slave AP to determine orestimate synchronization information. In one approach, the steeringcontroller 225 of the slave AP computes steering vectors for a jointtransmission of the first steered frame to the indicated stationdevices. The slave trigger frame may be decoded by some or all slaveAPs. In some embodiments, the steering controller 225 of the slave APestimates 1315 synchronization information for the first steered frame.For example, the steering controller 225 of the slave AP measures CFO,SFO, PO, TO, a reference value for PO, and/or a reference value for TOrelative to master AP, and synchronizes with the master AP for the firststeered frame. In some embodiments, PO, TO, or a combination of PO andTO is not determined during the sounding sequence, and the steeringcontroller 225 of the slave AP may determine, in the step 1315, a changein PO and/or TO between the sounding sequence and the steering sequenceas described above with respect to Eq. (1)-Eq. (4). In some embodiments,the station device may or may not decode the slave trigger frame.

In some embodiments, the steering controller 225 of the master AP causesits transceiver 228 to transmit 1320 the first steered frame includingcontent data to the station device, for example, after an IFS followingthe end of the slave trigger frame. In some embodiments, the master APand the slave AP may not transmit during the IFS. In some embodiments,the steering controller 225 of the slave AP causes its transceiver 228to transmit 1322 the first steered frame to the station device, whilethe master AP transmits 1320 the first steered frame to the stationdevice. In some embodiments, the master AP and the slave AP jointlytransmit the first steered frame by applying respective steering vectorsindicated in the steering information. In one aspect, IFS amongdifferent frames may vary or change. In some embodiments, time allocatedto the first steered frame and the second steered frame may bedifferent. In some embodiments, the station device receives and decodes1328 the joint transmission of the content data.

In some embodiments, the station device transmits 1333 anacknowledgement frame, acknowledging a successful receipt of the firststeered frame. In some embodiments, different station devices maysequentially or jointly transmit the acknowledgement frame, for examplethrough UL-OFDMA or UL MU-MIMO. In some embodiments, the steeringcontroller 225 of the master AP and the steering controller 225 of theslave AP receive and schedule 1334, 1335 the joint transmission of thesecond steered frame. For example, steering controllers 225 of themaster AP and the slave AP decide which data to jointly transmit in thesecond steered frame as well as the MCS/Nss to use for encoding (e.g.,whether to re-transmit un-ACKed frames at a different MCS, etc.), basedon a previously negotiated policy, according to the ACK frame received.

In some embodiments, the steering controller 225 of the master APconfigures or causes its transceiver to transmit 1340 a slave NDP frameto the slave AP, for example, after an IFS following acknowledgement. Insome embodiments, the IFS before the NDP frame may be SIFS (e.g., 16 μs)or less. In one aspect, the slave NDP frame includes a preamble andpossibly extra LTFs, with no content data. In one aspect, the slave NDPframe serves as the mid-amble of FIG. 10 or FIG. 11. In one aspect, theslave trigger frame instructs, causes, or allows the slave AP todetermine or estimate synchronization information. In some embodiments,the steering controller 225 of the slave AP estimates 1345synchronization information for a second steered frame. For example, thesteering controller 225 of the slave AP measures or estimates CFO, SFO,PO, TO, a reference value for PO, and/or a reference value for TOrelative to master AP, and synchronizes with the master AP for thesecond steered frame according to the CFO, SFO, PO, TO, a referencevalue for PO, and/or a reference value for TO. In some embodiments. PO,TO, or a combination of PO and TO is not determined during the soundingsequence, and the steering controller 225 of the slave AP may determine,in the step 1345, a change in PO and/or TO between the sounding sequenceand the steering sequence as described above with respect to Eq. (1)-Eq.(4). In some embodiments, the station device may or may not decode theslave trigger frame. 10146 j In some embodiments, the steeringcontroller 225 of the master AP causes its transceiver 228 to transmit1350 the second steered frame including content data to the stationdevice, for example, after an IFS following the end of the slave NDPframe. In some embodiments, time allocated for the second steered frameis longer than time allocated for the first steered frame. In someembodiments, the IFS may be SIFS (e.g., 16 μs) or less. In someembodiments, the master AP and the slave AP may not transmit during theIFS. In some embodiments, the steering controller 225 of the slave APcauses its transceiver 228 to transmit 1352 the second steered frame tothe station device, while the master AP transmits 1350 the secondsteered frame to the station device. In some embodiments, the master APand the slave AP jointly transmit the second steered frame by applyingrespective steering vectors indicated in the steering information. Insome embodiments, the station device receives and decodes 1358 the jointtransmission of the content data.

In some embodiments, the slave AP performs synchronization with themaster AP for a joint transmission, for example, for sounding, steering,or both. In one aspect, the slave APs are configured to maintain areference phase/timing offset (PO_ref and TO_ref, or H_ref) relative tomaster AP as measured prior to joint sounding, and compare with thephase/timing offset observed prior to steering (PO_new and TO_new, orH_new). Any difference between the two may be compensated during jointsteering transmission. However, there may be a potential time lag T1between measurement of PO_ref, TO_ref, H_ref and actual joint sounding.Likewise, there may be a time lag T2 between measurement of PO_new,TO_new, H_new and actual joint steering. The two lags T1 and T2 may bedifferent. During these lags, the phase-offset and timing-offsetrelative to master AP may drift (e.g., due to CFO/SFO), because the lagsmay be unequal. When compensating for phase/timing offsets, the slaveAPs may account for any drift that might have occurred between the timeof measurement of these offsets and the time of joint TX (e.g., bycompensating for CFO/SFO accurately from the time of measurement untilthe joint TX of sounding or steered frame has completed).

In some embodiments, slave APs may refine their CFO/SFO/PO/TO relativeto the master AP in various ways. In one approach, slave APs may refineCFO/SFO/PO/TO according to a NDP frame transmitted by the master APcontaining a variable number of LTFs (signaled in preamble).

In another approach, slave APs may refine CFO/SFO/PO/TO according topiggy backed training frames. For example, the master AP may designateany frame that it transmits for CFO/SFO/PO/TO refinement. The master APmay indicate in the preamble of the frame that the frame is for theCFO/SFO/PO/TO refinement, for example, according to a BSS color. In oneapproach, the master AP may embed additional LTFs that may bediscontiguous. For example, the master AP transmits two or more LTFsymbol groups that are separated by at least a symbol. In one example, afirst LTF symbol group is located in a preamble of a data packet, and asecond LTF symbol group is located at the end of the data packet, with aspacing corresponding to at least a symbol between the first LTF symbolgroup and the second LTF symbol group. In one aspect, a payload of theframe for CFO/SFO/PO/TO refinement may be encoded, but the slave APs maystill perform CFO/SFO/PO/TO correction or estimation based on thepreamble of the frame transmitted by the master AP.

In another approach, slave APs may refine CFO/SFO/PO/TO according to afeedback from station devices. For example, station devices can provideas feedback the measured CFO/SFO/PO/TO (per AP) from the master AP andthe slave APs, or a difference of the slave AP's offsets relative tomaster AP. The station devices can track this via specialized LTFs orpilots in the joint steered frame (e.g., in 80 MHz, 16 pilots can besplit among 4 APs with 4 pilots per AP as a simple scheme). In someembodiments, the slave APs receive the estimates from the STAs, andapply the received estimates to re-sync their CFO/SFO/PO/TO relative tomaster. In one aspect, the estimates from multiple STAs can help improvethe quality of the estimation.

In some embodiments, the master AP can only be heard by some of theslave APs while the rest of the slave APs can be heard by the slave APsthat hear the master AP. In this case, the scheme can be extended byrepeating the slave trigger twice or more. For example, the firsttransmission may be sent only by the master AP and the slave APs. Theslave APs that heard the master AP may join the master AP in the nexttransmission. For example, the slave AP that has RSSI above a threshold(e.g. −70 dBm) can join the master AP in the subsequent slave triggertransmission. The master AP and the slave AP that heard the master APmay jointly send a second transmission. The slave AP may perform CFO/SFOpre-corrections as described above. Additional APs may hear the jointtransmission by the master AP and the slave AP, and the additional APsmay participate in the next joint transmission.

In some embodiments, mid-ambles are utilized to allow to correct oradjust synchronization, for example, as described above with respect toFIGS. 10 and 11. In some embodiments, instead of implementingmid-ambles, the slave AP may track the master AP during jointtransmission by all APs. The slave AP may track the master AP when theslave AP supports receiving on one channel while transmitting onanother. In some embodiments, the transmission channel is used forcommunication to a station device (e.g., joint AP transmission) whilethe receiving channel is used by the slave AP for tracking the master APas well as potentially for receiving new data from the master AP to betransmitted jointly to the station device in either subsequent packetsor during the current packet after some fixed or known amount of timegreater than the highest decoding delay of all slave AP. In someembodiments, the same reference oscillator may be used for both channelshence tracking on the receiving channel enables the slave AP to removephase drift on the transmitting channel. Some small error may remain dueto uncorrelated random phase noise between the two channels, but thiserror may be small enough such that the mid-ambles can be omitted orbypassed.

In some embodiments, a first access point (e.g., master AP) includes afirst transceiver, a second transceiver, and a steering controller. Insome embodiments, the steering controller is configured to cause thefirst transceiver to communicate with (e.g., transmit to and/or receivefrom) a second access point (e.g., slave AP) through a first channelfrequency, synchronization information for a joint transmission by thefirst access point and the second access point, and cause the secondtransceiver to transmit to a station device through a second channelfrequency, a portion of a steered frame, while the second access pointtransmits, through the second channel frequency, the portion of thesteered frame to the station device according to the synchronizationinformation for the joint transmission. In some embodiments, thesynchronization information includes a carrier frequency offset or asampling frequency offset. In some embodiments, the second access pointis configured to synchronize with the first access point for the jointtransmission according to the synchronization information. In someembodiments, the first transceiver and the second transceiver share orare coupled to a same reference clock.

In some embodiments, an access point (e.g., slave AP) includes a firsttransceiver, a second transceiver, and a steering controller. In someembodiments, the steering controller is configured to cause the firsttransceiver to receive from another access point (e.g., master AP)through a first channel frequency, synchronization information for ajoint transmission by the access point and the another access point, andcause the second transceiver to transmit to a station device through asecond channel frequency, a portion of a steered frame according to thesynchronization information for the joint transmission, while theanother access point transmits, through the second channel frequency,the portion of the steered frame to the station device. In someembodiments, the synchronization information includes a carrierfrequency offset or a sampling frequency offset. In some embodiments,the first transceiver and the second transceiver share or are coupled toa same reference clock.

It should be noted that certain passages of this disclosure canreference terms such as “first” and “second” in connection with subsetsof transmit spatial streams, sounding frames, response, and devices, forpurposes of identifying or differentiating one from another or fromothers. These terms are not intended to merely relate entities (e.g., afirst device and a second device) temporally or according to a sequence,although in some cases, these entities can include such a relationship.Nor do these terms limit the number of possible entities (e.g.,beamformers and/or beamformees) that can operate within a system orenvironment. It should be understood that the systems described abovecan provide multiple ones of any or each of those components and thesecomponents can be provided on either a standalone machine or, in someembodiments, on multiple machines in a distributed system. In addition,the systems and methods described above can be provided as one or morecomputer-readable programs or executable instructions embodied on or inone or more articles of manufacture, e.g., a floppy disk, a hard disk, aCD-ROM, a flash memory card, a PROM, a RAM, a ROM, or a magnetic tape.The programs can be implemented in any programming language, such asLISP, PERL, C, C++, C#, or in any byte code language such as JAVA. Thesoftware programs or executable instructions can be stored on or in oneor more articles of manufacture as object code.

While the foregoing written description of the methods and systemsenables one of ordinary skill to make and use embodiments thereof, thoseof ordinary skill will understand and appreciate the existence ofvariations, combinations, and equivalents of the specific embodiment,method, and examples herein. The present methods and systems shouldtherefore not be limited by the above described embodiments, methods,and examples, but by all embodiments and methods within the scope andspirit of the disclosure.

We claim:
 1. A method for a multiple-input multiple-output (MIMO)communication, the method comprising: transmitting, during a first timeperiod, by a master access point to a slave access point, informationfor a joint transmission by the master access point and the slave accesspoint; causing, by the master access point, the slave access point toestimate synchronization information for the joint transmission,according to the information for the joint transmission; transmitting,during a second time period after the first time period, by the masteraccess point, a portion of a null data packet to a station device;causing, during the second time period, by the master access point, theslave access point to transmit the portion of the null data packet tothe station device, according to the synchronization information for thejoint transmission; and causing the station device to determine steeringinformation for the MIMO communication, according to the null datapacket.
 2. The method of claim 1, wherein causing the slave access pointto estimate the synchronization information includes causing the slaveaccess point to estimate a carrier frequency offset or a samplingfrequency offset with respect to the master access point.
 3. The methodof claim 1, further comprising: transmitting, during a third time periodbetween the first time period and the second time period, a null datapacket announcement to the station device, while the slave access pointtransmits the null data packet announcement to the station device duringthe third time period according to the synchronization information, thenull data packet announcement transmitted by the master access point andthe slave access point allowing the station device to prepare for thenull data packet.
 4. The method of claim 1, wherein the second timeperiod is immediately after the first time period.
 5. A first accesspoint for a multiple-input multiple-output (MIMO) communicationcomprising: a transceiver, and a soundings controller, the soundingscontroller configured to: cause, during a first time period, thetransceiver to transmit, to a second access point, information for ajoint transmission by the first access point and the second accesspoint, the information for the joint transmission allowing the secondaccess point to estimate synchronization information for the jointtransmission, and cause, during a second time period after the firsttime period, the transceiver to transmit a portion of a null data packetto a station device, while the second access point transmits the portionof the null data packet to the station device according to thesynchronization information, the joint transmission of the portion ofthe null data packet by the first access point and the second accesspoint allowing the station device to determine steering information forthe MIMO communication.
 6. The first access point of claim 5, whereinthe soundings controller is further configured to cause the transceiverto: transmit, during a third time period before the first time period, aslave trigger frame to the second access point, the slave trigger frameallowing the second access point to estimate additional synchronizationinformation for a null data packet announcement, and transmit, during afourth time period between the third time period and the first timeperiod, the null data packet announcement to the station device, whilethe second access point transmits the null data packet announcement tothe station device during the fourth time period according to thesynchronization information, the null data packet announcementtransmitted by the first access point and the second access pointallowing the station device to prepare for the null data packet.
 7. Thefirst access point of claim 5, wherein the soundings controller isfurther configured to cause the transceiver to transmit the informationfor the joint transmission in a preamble of the null data packet.
 8. Thefirst access point of claim 5, wherein the soundings controller isfurther configured to cause the transceiver to transmit the informationfor the joint transmission in a null data packet announcement, the nulldata packet announcement allowing the station device to prepare for thenull data packet.
 9. The first access point of claim 8, wherein thesoundings controller is further configured to cause the transceiver totransmit, during a third time period between the first time period andthe second time period, a training field to the station device, whilethe second access point transmits the training field to the stationdevice during the third time period, the training field transmitted bythe first access point and the second access point allowing the stationdevice to adjust a gain setting to receive the portion of the null datapacket.
 10. The first access point of claim 5, wherein the soundingscontroller is further configured to cause the transceiver to: transmit,during a third time period between the first time period and the secondtime period, a null data packet announcement to the station device,while the second access point transmits the null data packetannouncement to the station device during the third time periodaccording to the synchronization information, the null data packetannouncement transmitted by the first access point and the second accesspoint allowing the station device to prepare for the null data packet.11. The first access point of claim 5, wherein the information for thejoint transmission includes two or more long training field (LTF) symbolgroups that are separated by at least a symbol.
 12. The first accesspoint of claim 5, wherein the second time period is immediately afterthe first time period.
 13. An access point for a multiple-inputmultiple-output (MIMO) communication comprising: a transceiver; and asoundings controller, the soundings controller configured to: cause thetransceiver to receive, during a first time period, from another accesspoint, information for a joint transmission by the access point and theanother access point, estimate synchronization information for the jointtransmission, according to the information for the joint transmission,and cause the transceiver to transmit, during a second time period afterthe first time period, a portion of a null data packet to a stationdevice according to the synchronization information, while the anotheraccess point transmits the portion of the null data packet to thestation device during the second time period, the portion of the nulldata packet transmitted by the access point and the another access pointallowing the station device to determine steering information for theMIMO communication.
 14. The access point of claim 13, wherein thesoundings controller is configured to estimate the synchronizationinformation for the joint transmission by estimating a carrier frequencyoffset, a sampling frequency offset, a first reference value for acommon phase offset, or a second reference value for a timing offsetwith respect to the another access point.
 15. The access point of claim13, wherein the soundings controller is further configured to: cause thetransceiver to receive, during a third time period before the first timeperiod, from the another access point, a slave trigger frame, andestimate additional synchronization information for a null data packetannouncement, according to the slave trigger frame, and cause thetransceiver to transmit, during a fourth time period between the thirdtime period and the first time period, the null data packet announcementto the station device according to the synchronization information,while the another access point transmits the null data packetannouncement to the station device during the fourth time period, thenull data packet announcement transmitted by the access point and theanother access point allowing the station device to prepare for the nulldata packet.
 16. The access point of claim 13, wherein the soundingscontroller is configured to cause the transceiver to receive theinformation for the joint transmission in a preamble of the null datapacket.
 17. The access point of claim 13, wherein the soundingscontroller is configured to cause the transceiver to receive theinformation for the joint transmission in a null data packetannouncement, the null data packet announcement allowing the stationdevice to prepare for the null data packet.
 18. The access point ofclaim 17, wherein the soundings controller is configured to cause thetransceiver to transmit, during a third time period between the firsttime period and the second time period, a training field to the stationdevice, while the another access point transmits the training field tothe station device during the third time period, the training fieldtransmitted by the access point and the another access point allowingthe station device to adjust a gain setting to receive the portion ofthe null data packet.
 19. The access point of claim 13, wherein thesoundings controller is configured to cause the transceiver to:transmit, during a third time period between the first time period andthe second time period, a null data packet announcement to the stationdevice, while the another access point transmits the null data packetannouncement to the station device during the third time periodaccording to the synchronization information, the null data packetannouncement transmitted by the access point and the another accesspoint allowing the station device to prepare for the null data packet.20. The access point of claim 13, wherein the second time period isimmediately after the first time period.